SCTLR and ACTLR are 32-bit for AArch32 and 64-bit for AArch64

AArch64 System register SCTLR_EL1[31:0] is architecturally mapped
to AArch32 System register SCTLR[31:0]
AArch64 System register ACTLR_EL1[31:0] is architecturally mapped
to AArch32 System register ACTLR[31:0].

`u_register_t` should be used when it's important to store the
contents of a register in its native size

Signed-off-by: Deepika Bhavnani <deepika.bhavnani@arm.com>
Change-Id: I0055422f8cc0454405e011f53c1c4ddcaceb5779