arm64: Document sysctls for emulated deprecated instructions

We have support for emulating a number of deprecated instructions in the
kernel with individual Kconfig options enabling this support per
instruction. In addition to the Kconfig options we also provide runtime
control via sysctls but this is not currently mentioned in the Kconfig so
not very discoverable for users. This is particularly important for
SWP/SWPB since this is disabled by default at runtime and must be enabled
via the sysctl, causing considerable frustration for users who have enabled
the config option and are then confused to find that the instruction is
still faulting.

Add a reference to the sysctls in the help text for each of the config
options, noting that SWP/SWPB is disabled by default, to improve the
user experience.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200625131507.32334-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>