ARM: 9281/1: improve Cortex A8/A9 errata help text

Document that !ARCH_MULTIPLATFORM is necessary because accessing
the the errata workaround registers may not work in non-secure
mode and mention that these erratas should be applied by the
bootloader instead.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>