arm:am33xx: Rework s_init and add board_early_init_f

arm:am33xx: Rework s_init and add board_early_init_f

With the changes to the i2c framework (and adopting the omap24xx_i2c
driver to them) we can no longer call i2c functions prior to gd having
been set and cleared.  When SPL booting, this is handled by setting gd
to point to SRAM in s_init.  However in the cases where we are loaded
directly by ROM (memory mapped NOR or QSPI) we need to make use of the
normal hooks to slightly delay these calls.

Signed-off-by: Tom Rini <trini@ti.com>