rp2040: Add spi slave example #215

Job	Run time
Unit Test Regz (ubuntu-latest)	30s
Build (ubuntu-latest)	24s
Unit Test Regz (windows-latest)	1m 18s
Build (windows-latest)	1m 29s
Unit Test Regz (macos-latest)	37s
Build Examples (espressif/esp)	2m 10s
Build (macos-latest)	28s
Build Examples (gigadevice/gd32)	2m 17s
Build Website	26s
Build Examples (microchip/avr)	2m 11s
Dry Run Packaging	1m 27s
Check that stm32 generated code is up to date	39s
Build Examples (microchip/atsam)	1m 57s
Build Examples (nordic/nrf5x)	2m 3s
Unit Test Ports (ubuntu-latest, gigadevice/gd32)	23s
Unit Test UF2 (ubuntu-latest)	15s
Build Examples (nxp/lpc)	2m 6s
Unit Test Ports (ubuntu-latest, raspberrypi/rp2xxx)	21s
Unit Test UF2 (windows-latest)	1m 14s
Build Examples (stmicro/stm32)	24s
Unit Test Ports (ubuntu-latest, stmicro/stm32)	27s
Build Examples (raspberrypi/rp2xxx)	22s
Unit Test UF2 (macos-latest)	31s
Unit Test Ports (windows-latest, gigadevice/gd32)	1m 41s
Build Examples (wch/ch32v)	2m 16s
Unit Test Ports (windows-latest, raspberrypi/rp2xxx)	59s
Unit Test Ports (windows-latest, stmicro/stm32)	1m 47s
Unit Test Ports (macos-latest, gigadevice/gd32)	34s
Unit Test Ports (macos-latest, raspberrypi/rp2xxx)	34s
Unit Test Ports (macos-latest, stmicro/stm32)	32s
	32m 22s

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