dts: arm: rpi_pico: remove #define from dts

Removing direct #define usage in the DTSI file and converting these
definitions to use a dt-bindings header instead.

Relocates the RPI_PICO_DEFAULT_IRQ_PRIORITY definition to a DTSI file and
introduces an override.dtsi file. The override file is used when no other
override file is present, allowing for better flexibility and compliance
with Zephyr’s DTS structure.

Fixes: #79719

Signed-off-by: Tarang Raval <tarang.raval@siliconsignals.io>