ARM: percpu: add SMP_ON_UP support

Permit the use of the TPIDRPRW system register for carrying the per-CPU
offset in generic SMP configurations that also target non-SMP capable
ARMv6 cores. This uses the SMP_ON_UP code patching framework to turn all
TPIDRPRW accesses into reads/writes of entry #0 in the __per_cpu_offset
array.

While at it, switch over some existing direct TPIDRPRW accesses in asm
code to invocations of a new helper that is patched in the same way when
necessary.

Note that CPU_V6+SMP without SMP_ON_UP results in a kernel that does not
boot on v6 CPUs without SMP extensions, so add this dependency to
Kconfig as well.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M