mmu: move huge_page_size to mmu.hh

No reason at all for page_size to be in mmu.hh but huge_page_size in mmu.cc.
Move it, so we can also use huge_page_size outside the mmu.cc scope.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>