Fix pcf for sram for icoboard

[splinedrive]

sram wasn't working for sram_addr >= 256*1024

[splinedrive]

I have used the schematic from icoboard r0.3 and the datasheet of 51216 sram

[janderholm]

In case anyone else encounters this.

There are references to HRAM_CK which also uses N10. So this patch alone does not compile.

I removed HRAM_CK from icosoc.py and that makes it compile. memtest seem to run with 1024KB SRAM.