rp23xx: add TRNG driver (hardware /dev/random and /dev/urandom)

[ricardgb]

Summary

The RP2350 (rp23xx) embeds an Arm CryptoCell-style true random number
generator — a sampled ring oscillator conditioned by von-Neumann, CRNGT and
autocorrelation health tests, presenting 192 bits in the six EHR_DATA words —
but there is no driver for it, so /dev/urandom on rp23xx is the software
xorshift PRNG.

This adds arch/arm/src/rp23xx/rp23xx_rng.c, a polling driver that serves the
conditioned hardware bits as /dev/random and /dev/urandom. Entropy is drawn
rarely (an mbedtls CTR_DRBG seeds once and reseeds occasionally), so a blocking
poll is used rather than interrupt plumbing; health-test failures discard the
block and re-arm the source, so only conditioned entropy is ever returned.

New CONFIG_RP23XX_TRNG selects ARCH_HAVE_RNG, which switches /dev/urandom
from the software PRNG to the hardware source (backing getrandom() and thus any
entropy consumer, e.g. mbedtls).

Impact

• rp23xx gains a real hardware entropy source; opt-in via CONFIG_RP23XX_TRNG.
• No change when the option is off (default n).

Testing

Tested on a Raspberry Pi Pico 2 W:

• /dev/random and /dev/urandom register from the TRNG;
• a 32-byte read returns at real ring-oscillator sampling latency (~10 ms);
• two independent 32-byte samples differ (live, non-stuck source);
• the CryptoCell CRNGT continuous test — which would stall a stuck source —
  passes, and mbedtls seeds its CTR_DRBG from /dev/urandom successfully.

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Disclosure: this change was prepared by an AI agent (Claude Code) at the
direction of the author, who reviewed and tested it on hardware before
submission.

🤖 Generated with Claude Code

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[ricardgb]

Thanks for the review @xiaoxiang781216 — all four points addressed in d2531b2:

• dropped the local MIN fallback and used the sys/param.h macro directly (added the include)
• n = MIN(len - got, RP23XX_TRNG_EHR_BYTES); and return got; casts removed as suggested
• sample[][] in the startup health check is no longer static (192 bytes on the stack)

Also fixed the check CI failures from the previous push (nxstyle long lines and codespell hits on "EHR"); tools/checkpatch.sh -c -u -m -g now passes locally on the commit.

(As before: this change was made with the help of an AI agent and human-reviewed before pushing.)