gpio: syscon: add soc specific callback to assign output value

Some SoCs (like Keystone) may require to perform special
sequence of operations to assign output GPIO value, so default
implementation of .set() callback from gpio-syscon driver
can't be used.

Hence, add optional, SoC specific callback to assign output
gpio value.

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