MLK-10038-1: mfd: si476x-i2c: Add support of si476x-rev4.0 board

Currently, si476x-rev1.0 and si476x-rev4.0 board just support A10 compatible
command set. For si476x-rev1.0 board, its firmware revision is unsupported and
will revert to A10 compatible function. For si476x-rev4.0 board, its firmware
revision is two and will use A30 function, but A30 command set function can't
work for the rev4.0 board.
So make the command set configurable in dts. If "revision-a10" is present,
set the revision to SI476X_REVISION_A10 to use A10 compatible commit set.
Otherwise, get the revision from si476x register.

Signed-off-by: Zidan Wang <b50113@freescale.com>