dts: arm: rp2040: Improve naming of included files

Rename rpi_pico_common.dtsi to rp2040_reset.h . This is more consistent
with the wider Zephyr source tree, and is foundation work ahead of
introducing the RP2350 SoC.

Add missing include guard. This shouldn't be required, but it is
consistent with other header files in the same directory.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>