ARM: 7436/1: Do not map the vectors page as write-through on UP systems

The vectors page has been traditionally mapped as WT on UP systems but
this creates a mismatched alias with the directly mapped RAM that is
using WB attributes. On newer processors like Cortex-A15 this has
implications on the data/instructions coherency at the point of
unification (usually L2).

This patch removes such restriction.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>