xen/arm: memaccess: Initialize correctly *access in __p2m_get_mem_access

xen/arm: memaccess: Initialize correctly *access in __p2m_get_mem_access

The commit 8d84e701fd "xen/arm: initialize access" initializes
*access using the wrong enumeration type. This result to a warning
using clang:

mem_access.c:50:20: error: implicit conversion from enumeration type
'p2m_access_t' to different enumeration type 'xenmem_access_t'
[-Werror,-Wenum-conversion]
    *access = p2m->default_access;
            ~ ~~~~~^~~~~~~~~~~~~~

The correct solution is to use the array memaccess that will do the
conversion between the 2 enums.

Fixes: 8d84e701fd ("xen/arm: initialize access")
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>