arm64: Introduce sysreg_clear_set()

Currently we have a couple of helpers to manipulate bits in particular
sysregs:

 * config_sctlr_el1(u32 clear, u32 set)

 * change_cpacr(u64 val, u64 mask)

The parameters of these differ in naming convention, order, and size,
which is unfortunate. They also differ slightly in behaviour, as
change_cpacr() skips the sysreg write if the bits are unchanged, which
is a useful optimization when sysreg writes are expensive.

Before we gain yet another sysreg manipulation function, let's
unify these with a common helper, providing a consistent order for
clear/set operands, and the write skipping behaviour from
change_cpacr(). Code will be migrated to the new helper in subsequent
patches.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>