ARMv7: Flush the vectors page using the base address

With the domains removal patch, the vectors page is populated using the
kernel linear mapping address. However, the flush_icache_range()
function must be called on the high vectors address because the I-cache
is allowed to have aliases (or can even be ASID-tagged VIVT). Note that
on ARMv7 processors the D-cache is non-aliasing.