xen/arm: Use check_workaround to handle the erratum 766422

xen/arm: Use check_workaround to handle the erratum 766422

Currently, Xen is accessing the stored MIDR everytime it has to check
whether the processor is affected by the erratum 766422.

This could take advantage of the new capability bitfields to detect
whether the processor is affected at boot time.

With this patch, the number of instructions to check the erratum is
going down from ~13 (including 2 loads and a co-processor access) to
~6 instructions (include 1 load).

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>