mtd: nand: pxa3xx: fix build on ARM64

mtd: nand: pxa3xx: fix build on ARM64

In preparation to enable ARCH_MMP on ARM64, a couple of fixes are needed
to build the pxa3xx_nand driver:

Legacy DMA will only used on ARM, so also make it condtional on
CONFIG_ARM.
__raw_{read,write}sl are not available on ARM64 or generically, so use
the readsl/writesl variants instead. Somewhat inconsistently,
{read,write}sl are inherently non-swapping with the generic version
using __raw_{read,write}l.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
[Brian: added one more __raw_readsl -> readsl]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>