MIPS: Export mm switching functions used by KVM

KVM makes use of check_switch_mmu_context(), check_mmu_context() &
get_new_mmu_context() which are no longer static inline functions in a
header. As such they need to be exported for KVM to successfully build
as a module, which was previously overlooked. Add the missing exports.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 4ebea49c ("MIPS: mm: Un-inline get_new_mmu_context")
Fixes: 42d5b846 ("MIPS: mm: Unify ASID version checks")