This adds driver support for the 12 GPIO banks on the STM32MP157.
As they are accessible to both the Cortex-A cores as well as the Cortex-M
core, modifications to these are protected by a hardware spinlock
and clocks are enabled/disabled as required.

All register fiddling done by the driver is collected in <soc/stm32/gpio.h>,
so future PBL code may make use of it as well to chainload barebox proper.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>