ARM: mm: Add support for flushing HugeTLB pages.

On ARM we use the __flush_dcache_page function to flush the dcache
of pages when needed; usually when the PG_dcache_clean bit is unset
and we are setting a PTE.

A HugeTLB page is represented as a compound page consisting of an
array of pages. Thus to flush the dcache of a HugeTLB page, one must
flush more than a single page.

This patch modifies __flush_dcache_page such that all constituent
pages of a HugeTLB page are flushed.

Signed-off-by: Steve Capper <steve.capper@linaro.org>
Reviewed-by: Will Deacon <will.deacon@arm.com>