ARC: mm: hack to allow 2 level build with 4 level code

PMD_SHIFT is mapped to PUD_SHIFT or PGD_SHIFT by asm-generic/pgtable-*
but only for !__ASSEMBLY__

tlbex.S asm code has PTRS_PER_PTE which uses PMD_SHIFT hence barfs
for CONFIG_PGTABLE_LEVEL={2,3} and works for 4.

So add a workaround local to tlbex.S - the proper fix is to change
asm-generic/pgtable-* headers to expose the defines for __ASSEMBLY__ too

Signed-off-by: Vineet Gupta <vgupta@kernel.org>