SPI: Change read/write buffer pointers to void *

There's no reason to require callers to cast their data to uint8_t *
when the data might e.g. originate in a packed struct or some other
data type. Instead, be nice to callers and let them use any pointer
they want. Additionally, declare the TX buffer as a const pointer so
unnecessary typecasts aren't needed for that either (if the data
originates in a const location).

Change-Id: I1482ca4e350b5a7fbda6871ed9f54f255af3aa9e
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>