New helper functions to get GPIO banks configuration from the FDT.

stm32_get_gpio_bank_pinctrl_node() allows stm32mp platforms to
differentiate specific GPIO banks when these are defined with a specific
path in the FDT.

fdt_get_gpio_bank_pin_count() returns the number of pins in a GPIO bank
as it depends on the SoC variant.

Change-Id: I4481774152b3c6bf35bf986f58e357c2f9c19176
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>