omap3: sniper: Convert to device-tree control and DM I2C

omap3: sniper: Convert to device-tree control and DM I2C

This converts the sniper board (LG P970) to device-tree control
and DM I2C, both for SPL and U-Boot.

Note that we lose the call to board_mmc_power_init to enable power
for MMC2. This is now expected to take place through proper
regulators, which are not yet available with the twl4030 driver.
The call to twl4030_power_mmc_init is moved to spl_board_init for now.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>