xen/arm: Extract MMU-specific MM code

xen/arm: Extract MMU-specific MM code

Currently, most of the code is in arm/mm.{c,h} and arm/arm64/mm.c
is MMU-specific. To make the MM code extendable, this commit extracts
the MMU-specific MM code.

Extract the boot CPU MM bringup code from arm/mm.c to mmu/setup.c.
While moving, mark pte_of_xenaddr() as __init to make clear that
this helper is only intended to be used during early boot.

Move arm/arm64/mm.c to arm/arm64/mmu/mm.c. Since the function
setup_directmap_mappings() has different implementations between
arm32 and arm64, move their arch-specific implementation to
arch-specific arm{32,64}/mmu/mm.c instead using #ifdef again.

For header files, move MMU-related function declarations in
asm/mm.h, declaration of global variable init_ttbr and the
declaration of dump_pt_walk() in asm/page.h to asm/mmu/mm.h

Also modify the build system (Makefiles in this case) to pick above
mentioned code changes.

Take the opportunity to fix the in-code comment coding styles when
possible, and drop the unnecessary #include headers in the original
arm/mm.c.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>