A71CH driver, UID as serial number

- Basic driver for A71CH. Resets device, negotiates transmission sizes,
and fetches chip UID.
- On EVT2 boards, probe A71CH and use UID as serial#.
- Add support for SMBus-style block reads to the i2c-gpio driver.

Change-Id: Ib47fb3f4a6e8d4a024877bf30916a4b195c7187e