regmap: mmio: add config option to allow relaxed MMIO accesses

On some platforms (eg armv7 due to the CONFIG_ARM_DMA_MEM_BUFFERABLE)
MMIO R/W operations always add memory barriers which can increase load,
decrease battery life or in general reduce performance unnecessarily
on devices which access a lot of configuration registers and where
ordering does not matter (eg. media accelerators like the Verisilicon /
Hantro video decoders).

Drivers used to call the relaxed MMIO variants directly but since they
are now accessing the MMIO registers via regmaps (to compensate for
different VPU HW reg layouts via regmap fields), there is a need for a
relaxed API / config to preserve existing behaviour.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Link: https://lore.kernel.org/r/20201014203024.954369-1-adrian.ratiu@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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