arm64: Kconfig: Enable LSE atomics by default

Now that we're seeing CPUs shipping with LSE atomics, default them to
'on' in Kconfig. CPUs without the instructions will continue to use
LDXR/STXR-based sequences, but they will be placed out-of-line by the
compiler.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>