arm64: Make DMA coherent and strongly ordered mappings not executable

pgprot_{dmacoherent,writecombine,noncached} don't need to generate
executable mappings with side-effects like __sync_icache_dcache() being
called when the mapping is in user space.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Tested-by: Laura Abbott <lauraa@codeaurora.org>
Tested-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Cc: <stable@vger.kernel.org>