arm64: Remove a redundancy in sysreg.h

This is really trivial; there is a dup (1 << 16) in the code

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Stefan Traby <stefan@hello-penguin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>