of/address: Add infrastructure to declare MMIO as non-posted

This implements the 'nonposted-mmio' boolean property. Placing this
property in a bus marks all direct child devices as requiring
non-posted MMIO mappings. If no such property is found, the default
is posted MMIO.

of_mmio_is_nonposted() performs this check to determine if a given
device has requested non-posted MMIO.

of_address_to_resource() uses this to set the IORESOURCE_MEM_NONPOSTED
flag on resources that require non-posted MMIO.

of_iomap() and of_io_request_and_map() then use this flag to pick the
correct ioremap() variant.

This mechanism is currently restricted to builds that support Apple ARM
platforms, as an optimization.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>