ARM: use ARM_DMA_ZONE_SIZE to adjust the zone sizes

Rather than each platform providing its own function to adjust the
zone sizes, use the new ARM_DMA_ZONE_SIZE definition to perform this
adjustment.  This ensures that the actual DMA zone size and the
ISA_DMA_THRESHOLD/MAX_DMA_ADDRESS definitions are consistent with
each other, and moves this complexity out of the platform code.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>