ARM: 8693/1: discard memblock arrays when possible

On ARM the generic pfn_valid() version is used with some configurations
such as SA1100 based devices. In that case the memblock arrays are no
longer used after boot and can be discarded.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>