regmap: mmio: Convert to regmap_bus and fix accessor usage

Currently regmap-mmio uses the __raw accessors to read and write from
memory.  This is not safe as these interact poorly with spinlocks and
are not guaranteed to generate emulated instructions on at least ARM
where regmap is commonly used.  The APIs that are provided all provide
some byte swapping so this is difficult to do with the current
regmap-mmio implementation which attempts to use the regmap core byte
swapping.

We can fix this by modernising the MMIO implementation to use
reg_read() and reg_write() operations which were added after the API was
implemented and pass simple unsigned integers through to the bus, making
use of the formatting provided by the I/O accessors using a similar
pattern to that used by the core.  This will be less efficient for block
I/O operations since we now enable and disable any required clocks per
register but it is not clear that any users of regmap-mmio actually use
block I/O and there is room to optimise later.

This removes support for big endian I/O on 64 bit registers since no I/O
accessors are provided, no current users were found and support can be
added easily once they are available.

In addition make the default endianness little endian.  This was the
behaviour prior to 29bb45f2 (regmap-mmio: Use native endianness
for read/write) and is the behaviour desired by most existing users, the
users have been audited and those that need native endianness converted
to request it explicitly.  Previously native was documented as the
default but due to the byte swapping in the accessors this was not
correctly implemented.

Fixes: 29bb45f2 (regmap-mmio: Use native endianness for read/write)
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Mark Brown <broonie@kernel.org>