ti: remove usage of DM_I2C_COMPAT and don't disable DM_I2C in SPL

DM_I2C_COMPAT is a compatibility layer that allows using the non-DM I2C
API when DM_I2C is used. The goal is to eventually remove DM_I2C_COMPAT
when all I2C "clients" have been migrated to use the DM API.
This a step in that direction for the TI based platforms.
Build tested with buildman:
buildman -dle am33xx ti omap3 omap4 omap5 davinci keystone

boot tested with:
am335x_evm, am335x_boneblack, am335x_boneblack_vboot (DM version),
am57xx_evm, dra7xx_evm, k2g_evm, am437x_evm

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>