ARM: decompressor: avoid speculative prefetch from protected regions

Currently first 1GB memory mapped with read, write and execute
permissions and unfortunately speculative prefetch causing
undesirable sideffects if those regions are protected.

Hence the flag CONFIG_ARM_DECOMPRESSOR_LIMIT limit the size
of the memory to be mapped by decompressor with read, write
and execute permissions to avoid speculative prefetch.

Change-Id: I3f2495649153ae16040579b71dff5b78315b29a8
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>