ti: armv7: Move SPL SDRAM init to the right place, drop unused CONFIG_SPL_STACK

Currently in some cases SDRAM init requires global_data to be available
and soon this will not be available prior to board_init_f().  Adjust the
code paths in these cases to be correct.  In some cases we had the SPL
stack be in DDR as we might have large stacks (due to Falcon Mode +
Environment).  In these cases switch to CONFIG_SPL_STACK_R.  In other
cases we had simply been setting CONFIG_SPL_STACK into SRAM.  In these
cases we no longer need to (CONFIG_SYS_INIT_SP_ADDR is used and is also
in SRAM) so drop those lines.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested on Beagleboard, Beagleboard xM
Tested-by: Matt Porter <mporter@konsulko.com>
Tested on Beaglebone Black, AM43xx GP EVM, OMAP5 uEVM, OMAP4 Pandaboard
Tested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>