arm/mem_access: Introduce BIT_ULL bit operation

arm/mem_access: Introduce BIT_ULL bit operation

We introduce the BIT_ULL macro to using values of unsigned long long as
to enable setting bits of 64-bit registers on AArch32.  In addition,
this commit adds a define holding the register width of 64 bit
double-word registers. This define simplifies using the associated
constants in the following commits.

Signed-off-by: Sergej Proskurin <proskurin@sec.in.tum.de>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>