i3100: add smbus_write_byte()

Required for Supermicro X7DB8, which needs the FBDIMM clock generator
setup during romstage.

Change-Id: I30ca8354087e851487aee0614595782131d4d9bc
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/1116
Tested-by: build bot (Jenkins)
3 files changed