iosys-map: Add per-word write

Like was done for read, provide the equivalent for write. Even if
current users are not in the hot path, this should future-proof it.

v2:
  - Remove default from _Generic() - callers wanting to write more
    than u64 should use iosys_map_memcpy_to()
  - Add WRITE_ONCE() cases dereferencing the pointer when using system
    memory
v3:
  - Fix precedence issue when casting inside WRITE_ONCE(). By not using ()
    around vaddr__ the offset was not part of the cast, but rather added
    to it, producing a wrong address
  - Remove compiletime_assert() as WRITE_ONCE() already contains it

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Reviewed-by: Christian König <christian.koenig@amd.com> # v1
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220628191016.3899428-2-lucas.demarchi@intel.com