gpio: stm32f7: Fix gpio bank hole management

In case "gpio-ranges" property is not present in device tree,
use default value for gpio_count and gpio_range.
This fixes an issue on stm32 F7 and H7 boards where "pinmux status -a"
command didn't return any pin status due to the fact that both stm32 F7
and H7 board DT doesn't use the gpio-ranges property.

Fixes: dbf928dd("gpio: stm32f7: Add gpio bank holes management")

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>