MIPS: Enable hugepage support for MIPS64r6

Our hugepage support already exists for MIPS64 CPUs, and is already
enabled for older architecture revisions. There's nothing MIPSr6
specific involved, and our hugepage support already works fine for
MIPS64r6 CPUs such as the I6500, so allow it to be selected in Kconfig.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org