xen/arm64: head: Introduce macros to create table and mapping entry

xen/arm64: head: Introduce macros to create table and mapping entry

At the moment, any update to the boot-pages are open-coded. This is
making more difficult to understand the logic of a function as each
update roughly requires 6 instructions.

To ease the readability, two new macros are introduced:
    - create_table_entry: Create a page-table entry in a given table.
    This can work at any level.
    - create_mapping_entry: Create a mapping entry in a given table.
    None of the users will require to map at any other level than 3rd
    (i.e page granularity). So the macro is only supporting 3rd level
    mapping.

Furthermore, the two macros are capable to work independently of the
state of the MMU.

Lastly, take the opportunity to replace open-coded version in
setup_fixmap() by the two new macros. The ones in create_page_tables()
will be replaced in a follow-up patch.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>