[ARM] fix constant values of MAX_DMA_ADDRESS

Since 8d5796d2ec6b5a4e7a52861144e63af438d6f8f7, we have allowed
PAGE_OFFSET to be configurable, so a constant virtual address
for MAX_DMA_ADDRESS is buggy.  It should be defined in terms of
PAGE_OFFSET rather than a constant virtual address.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>