mtd: phram: Allow cached mappings

Currently phram always uses ioremap(), but this is unnecessary when
normal memory is used.  If the reserved-memory node does not specify the
no-map property, indicating it should be mapped as system RAM and
ioremap() cannot be used on it, use a cached mapping using
memremap(MEMREMAP_WB) instead.

On one of my systems this improves read performance by ~70%.

(Note that this driver has always used normal memcpy/memset functions on
memory obtained from ioremap(), which sparse doesn't like.  There is no
memremap() variant which maps exactly to ioremap() on all architectures,
so that behaviour of the driver is not changed to avoid affecting
existing users, but the sparse warnings are suppressed in the moved code
with __force.)

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220510151822.1809278-1-vincent.whitchurch@axis.com