Change data type of vco-frequency-hz to a 64 bit

[omaganap]

linux/drivers/iio/frequency/ltc6952.c

Line 829 in b2f6d34

ret = of_property_read_u32(np, "adi,vco-frequency-hz",

[omaganap]

@nunojsa, should I make a pull request for this? or is there an open pull request that this can be added to?