am33xx/omap4+: Move SRAM_SCRATCH_SPACE_ADDR to <asm/arch/omap.h>

The location of valid scratch space is dependent on SoC, so move that
there.  On OMAP4+ we continue to use SRAM_SCRATCH_SPACE_ADDR.  On
am33xx/ti814x we want to use what the ROM defines as "public stack"
which is the area after our defined download image space.  Correct the
comment about and location of CONFIG_SPL_TEXT_BASE.

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