[ARM] ARMv7 errata: only apply fixes when running on applicable CPU

Currently, whenever an erratum workaround is enabled, it will be
applied whether or not the erratum is relevent for the CPU.  This
patch changes this - we check the variant and revision fields in the
main ID register to determine which errata to apply.

We also avoid re-applying erratum 460075 if it has already been applied.
Applying this fix in non-secure mode results in the kernel failing to
boot (or even do anything.)

This fixes booting on some ARMv7 based platforms which otherwise
silently fail.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>