am33xx: move ti i2c baseboard header handling to board/ti/am335x/

The i2c header is specific to ti(-derived) boards, and not generic for all
am335x boards.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>
[trini: Make re-apply with rtc32k_enable() applied]
Signed-off-by: Tom Rini <trini@ti.com>