aarch64: Use 64-bit reads/writes to access SCTLR_EL1

aarch64: Use 64-bit reads/writes to access SCTLR_EL1

We were previously inadvertently clearing the top 32 bits of SCTLR_EL1
during read_memory/write_memory as a result of using 32-bit operations
to access the register and because the fields used to temporarily
store the register were 32-bit. Fix it.

Change-Id: I657d7f949e1f7ab6bf90609e3f91cae09cade31a
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7939
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>