exynos5420: Don't map low addresses that lead nowhere

I just spent half a day (including the time to implement a stack dumper)
to figure out that I am reading from a NULL pointer. A problem this
simple should be more easy to catch. Let's mark the address range below
SRAM as uncached so that the MMU can yell at you right away for being
the bad programmer you are when you access a NULL pointer.

BUG=None
TEST=Manual

Change-Id: I4a3a13f75bf21b25732be2ecb69d47503eff1b53
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/170112
Reviewed-by: Ronald Minnich <rminnich@chromium.org>
2 files changed