UPSTREAM: 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>

Bug: 31432001
Change-Id: I81c503219e93dcfe83f4ce7d56b00b78150dfeb4
(cherry picked from commit 870828e5)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>