ARM: 7168/1: use cache type functions for arch_get_unmapped_area

There are already cache type decoding functions, so use those instead
of custom decode code which only works for ARMv6.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>