arm64: Use include/asm-generic/io.h

Include the generic I/O header file so that duplicate implementations
can be removed. This will also help to establish consistency across more
architectures regarding which accessors they support.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v5:
- fixup __raw_writew() -> __raw_writel() in iowrite32be()

Changes in v4:
- define xlate_dev_{kmem,mem}_ptr() before asm-generic/io.h include

Changes in v3:
- add io{read,write}{16,32}be() to match what 32-bit ARM does