arm:am33xx: Make dram_init call sdram_init() in some contexts

arm:am33xx: Make dram_init call sdram_init() in some contexts

We have two contexts for booting these platforms.  One is SPL which is
roughly: reset, cpu_init_crit, lowlevel_init, s_init, sdram_init, _main,
board_init_f from SPL, ... then U-Boot loads.  The other is a
memory-mapped XIP case (NOR or QSPI) where we do not run an SPL.  In
this case we go, roughly: reset, cpu_init_crit, lowlevel_init, s_init,
_main, regular board_init_f.

In the first case s_init will set a valid gd and then be able to call
sdram_init which in many cases will need i2c (which needs a valid gd for
gd->cur_i2c_bus).  In this second case we must (and are able to and
should) defer sdram_init() into dram_init() called by board_init_f as gd
will have been set in _main and cleared in board_init_f.

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