arm64: Kconfig: Tidy up errata workaround help text

The nature of silicon errata means that the Kconfig help text for our
various software workarounds has been written by many different people.
Along the way, we've accumulated typos and inconsistencies which make
the options needlessly difficult to read.

Fix up minor issues with the help text.

Signed-off-by: Will Deacon <will.deacon@arm.com>