gpio: syscon: retrive syscon node and regs offsets from dt

This patch adds handling of new "gpio,syscon-dev" DT property,
which allows to specify syscon node and data/direction registers
offsets in DT.

"gpio,syscon-dev" has following format:
	gpio,syscon-dev = <&syscon_dev data_reg_offset [direction_reg_offset]>;

where
 - syscon_dev - phandle on syscon node
 - data_reg_offset - offset of data register (in bytes)
 - direction_reg_offset - offset of dirrection register (optional, in bytes)

for example:
	gpio,syscon-dev = <&devctrl 0x254>;

In such way, the support of multiple Syscon GPIO devices is added.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>