LoongArch: Mark {dmw,tlb}_virt_to_page() exports as non-GPL

Mark {dmw,tlb}_virt_to_page() exports as non-GPL, in order to let
out-of-tree modules (e.g. OpenZFS) be built without errors. Otherwise
we get:

ERROR: modpost: GPL-incompatible module zfs.ko uses GPL-only symbol 'dmw_virt_to_page'
ERROR: modpost: GPL-incompatible module zfs.ko uses GPL-only symbol 'tlb_virt_to_page'

Reported-by: Haowu Ge <gehaowu@bitmoe.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>