drivers/gpio: stm32 fix gpio device init prio

STM32 GPIO device initialization priority was set to
CONFIG_KERNEL_INIT_PRIORITY_DEFAULT
Following introduction of __ZEPHYR_SUPERVISOR__ macro (#6835),
drivers intialization order have been reordered which blocks
some use cases now.
Increase GPIO init priority to get it initialized before SPI
and avoid a deadlock.

Fixes #7663


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>