rp2040: Add spi slave example #217

Job	Run time
Unit Test UF2 (ubuntu-latest)	16s
Check that stm32 generated code is up to date	37s
Unit Test UF2 (windows-latest)	57s
Build (ubuntu-latest)	24s
Unit Test UF2 (macos-latest)	29s
Unit Test Regz (ubuntu-latest)	18s
Build (windows-latest)	1m 23s
Unit Test Regz (windows-latest)	2m 3s
Build (macos-latest)	29s
Unit Test Regz (macos-latest)	57s
Build Examples (espressif/esp)	2m 26s
Build Website	19s
Build Examples (gigadevice/gd32)	2m 10s
Unit Test Ports (ubuntu-latest, gigadevice/gd32)	16s
Dry Run Packaging	1m 26s
Build Examples (microchip/avr)	2m 17s
Build Examples (microchip/atsam)	1m 59s
Unit Test Ports (ubuntu-latest, raspberrypi/rp2xxx)	21s
Build Examples (nordic/nrf5x)	1m 59s
Unit Test Ports (ubuntu-latest, stmicro/stm32)	22s
Build Examples (nxp/lpc)	1m 54s
Unit Test Ports (windows-latest, gigadevice/gd32)	55s
Build Examples (stmicro/stm32)	23s
Unit Test Ports (windows-latest, raspberrypi/rp2xxx)	1m 10s
Build Examples (raspberrypi/rp2xxx)	2m 26s
Unit Test Ports (windows-latest, stmicro/stm32)	1m 41s
Build Examples (wch/ch32v)	2m 15s
Unit Test Ports (macos-latest, gigadevice/gd32)	30s
Unit Test Ports (macos-latest, raspberrypi/rp2xxx)	48s
Unit Test Ports (macos-latest, stmicro/stm32)	37s
	34m 7s

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