arm64: kernel: Move config_sctlr_el1

Later patches need config_sctlr_el1 to set/clear bits in the sctlr_el1
register.

This patch moves this function into header a file.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>