serial: clps711x: Use mctrl_gpio helpers for handling modem signals

CLPS711X serial driver uses the system wide registers to control the
modem signals. Now gpio-syscon driver can be used for this purposes.
mctrl_gpio helpers allow us to create GPIO bindings for any of modem/tty
control signals that extends the functionality of the driver.
This patch makes such change.

This change does not break any current DT bindings, since DT support
for this platform is not introduced yet.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>