am57xx: remove non-DM I2C code

am57xx configs uses DM_I2C both in SPL and u-boot.
Remove code for non-DM I2C support.

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