soc: silabs: siwx91x: Rename pins

The reference manual and the schematics of the reference designs
numbers the pins from 0 to 57. So let's also apply this scheme to the
pinctrl.

Note while PIN are numbered between 0 and 64, the GPIOs are represented
with 4 block of 16 GPIOs. This is the way the GPIOs are documented in
the reference manual, so we don't change that (In addition,
gpio_port_pins_t is a uint32_t, so it won't be possible to represent 64
GPIOs in one gpio_port_pins_t anyway).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>