xen/arm: p2m: Re-implement p2m_set_mem_access using p2m_{set,get}_entry

xen/arm: p2m: Re-implement p2m_set_mem_access using p2m_{set,get}_entry

The function p2m_set_mem_access can be re-implemented using the generic
functions p2m_get_entry and __p2m_set_entry.

Also the function apply_p2m_changes is dropped completely as it is not
used anymore.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>