MIPS: ralink: Add missing symbol for highmem support.

MT7621 has highmem. this was previously not working as the required symbol
was not selected in the Kconfig file.

Signed-off-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14901/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>