ARM: fix executability of CMA mappings

The CMA region was being marked executable:

0xdc04e000-0xdc050000           8K     RW x      MEM/CACHED/WBRA
0xdc060000-0xdc100000         640K     RW x      MEM/CACHED/WBRA
0xdc4f5000-0xdc500000          44K     RW x      MEM/CACHED/WBRA
0xdcce9000-0xe0000000       52316K     RW x      MEM/CACHED/WBRA

This is mainly due to the badly worded MT_MEMORY_DMA_READY symbol, but
there are also a few other places in dma-mapping which should be
corrected to use the right constant.  Fix all these places:

0xdc04e000-0xdc050000           8K     RW NX     MEM/CACHED/WBRA
0xdc060000-0xdc100000         640K     RW NX     MEM/CACHED/WBRA
0xdc280000-0xdc300000         512K     RW NX     MEM/CACHED/WBRA
0xdc6fc000-0xe0000000       58384K     RW NX     MEM/CACHED/WBRA

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