powerpc/pkey: Define functions as static

Sparse tool is showing some warnings on pkeys.c file, mainly related to
storage class identifiers. There are static variables and functions not
declared as such. The same thing happens with an extern function, which
misses the header inclusion.

	arch/powerpc/mm/pkeys.c:14:6: warning: symbol 'pkey_execute_disable_supported' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:16:6: warning: symbol 'pkeys_devtree_defined' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:19:6: warning: symbol 'pkey_amr_mask' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:20:6: warning: symbol 'pkey_iamr_mask' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:21:6: warning: symbol 'pkey_uamor_mask' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:22:6: warning: symbol 'execute_only_key' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:60:5: warning: symbol 'pkey_initialize' was not declared. Should it be static?
	arch/powerpc/mm/pkeys.c:404:6: warning: symbol 'arch_vma_access_permitted' was not declared. Should it be static?

This patch fix al the warning, basically turning all global variables that
are not declared as extern at asm/pkeys.h into static.

It also includes asm/mmu_context.h header, which contains the definition of
arch_vma_access_permitted.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>