am33xx: Correct NON_SECURE_SRAM_START/END

Prior to Sricharan's cleanup of the boot parameter saving code, we
did not make use of NON_SECURE_SRAM_START on am33xx, so it wasn't a
problem that the address was pointing to the middle of our running SPL.
Correct to point to the base location of the download image area.
Increase CONFIG_SPL_TEXT_BASE to account for this scratch area being
used.  As part of correcting these tests, make use of the fact that
we've always been placing our stack outside of the download image area
(which is fine, once the downloaded image is run, ROM is gone) so
correct the max size test to be the ROM defined top of the download area
to where we link/load at.

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

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Changes in v2:
- Fix typo noted by Peter Korsgaard