drivers: gpio: Refactor drivers to use shared init priority

Refactors all of the on-chip GPIO drivers to use a shared driver class
initialization priority configuration, CONFIG_GPIO_INIT_PRIORITY, to
allow configuring GPIO drivers separately from other devices. This is
similar to other driver classes like I2C and SPI.

Most drivers previously used CONFIG_KERNEL_INIT_PRIORITY_DEFAULT or
CONFIG_KERNEL_INIT_PRIORITY_DEVICE, therefore the default for this new
option is the lower of the two, which means earlier initialization.

Driver-specific options for off-chip I2C- or SPI-based GPIO drivers are
left intact because they often need to be initialized at a different
priority than on-chip GPIO drivers.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>