ARM: Optionally allow ARMv6 to use 'normal, bufferable' memory for DMA

Provide a configuration option to allow the ARMv6 to use normal
bufferable memory for coherent DMA.  This option is forced to 'y'
for ARMv7, and offered as a configuration option on ARMv6.

Enabling this option requires drivers to have the necessary barriers
to ensure that data in DMA coherent memory is visible prior to the
DMA operation commencing.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>