powerpc/32s: move CTX_TO_VSID() into mmu-hash.h

In order to reuse it in switch_mmu_context(), this
patch moves CTX_TO_VSID() macro into asm/book3s/32/mmu-hash.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/26b36ef2939234a04b37baf6ffe50cba81f5d1b7.1622708530.git.christophe.leroy@csgroup.eu