arch: Fix Aarch64 WRITE_SYSREG macro width mismatch warning in Clang
Add 64-bit operand modifier to Aarch64 WRITE_SYSREG macro inline assembler instruction to fix width mismatch warning produced when using Clang toolchain.
Signed-off-by: Marcos Agra-Trillo Marcos.Agra-Trillo@arm.com