pinctrl-sx150x: Replace sx150x_*_cfg by means of regmap API

The difference between 8 and 16 pin GPIO expanders can be accomodated by
the means of regmap API without resorting to using driver-specific
read/write accessors. This change, IMHO, brings the following benefits:

	- Replaces driver's idiosyncratic way of dealing with
	  mult-register fields with regmap API, which, hopefuly,
	  makes the code a bit easier for a new reader to understand

	- Removes various multi-read for-loop register read logic
	  from various places in the code and puts it in a signle
	  place

	- Removes ad-hoc IRQ register caching code in
	  sx150x_irq_bus_sync_unlock, since that functionality is
	  provided by regmap

Besided aforementioned benefits this change also implements necessary
RegSense byte swap necessary for SX1503 and SX1506 variants of the chip.
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>