dts: arm: st: correct npgios for all stm32 gpio controllers

For almost all STM32 GPIO controllers, the number of supported GPIO
pins managed by a single controller is 16 (with some exceptions for
fewer). However, the default for ngpios in the device tree bindings
for gpio-controllers is 32; leading to inaccuracies in handling GPIO
for these controllers, such as presenting too many GPIOs in the GPIO
shell. This patch redefines the default for ngpios for "st,stm32-gpio"
compatible devices to 16 and adds the correct ngpios for the few
exceptions Zephyr current supports.

Signed-off-by: Michael R Rosen <mrrosen@alumni.cmu.edu>