MIPS: Allow storing pgd in C0_CONTEXT for MIPSr6

MIPS: Allow storing pgd in C0_CONTEXT for MIPSr6

CONFIG_MIPS_PGD_C0_CONTEXT, which allows a pointer to the page directory
to be stored in the cop0 Context register when enabled, was previously
only allowed for MIPSr2. MIPSr6 is just as able to make use of it, so
allow it there too.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16204/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>