rp2040: Add spi slave example #221

Job	Run time
Dry Run Packaging	1m 27s
Build Website	21s
Build Examples (espressif/esp)	2m 12s
Build Examples (gigadevice/gd32)	2m 11s
Build (ubuntu-latest)	33s
Check that stm32 generated code is up to date	45s
Build Examples (microchip/avr)	2m 18s
Unit Test Ports (ubuntu-latest, gigadevice/gd32)	17s
Build (windows-latest)	2m 10s
Build Examples (microchip/atsam)	1m 57s
Unit Test Ports (ubuntu-latest, raspberrypi/rp2xxx)	25s
Build (macos-latest)	34s
Build Examples (nordic/nrf5x)	2m 7s
Unit Test Ports (ubuntu-latest, stmicro/stm32)	27s
Build Examples (nxp/lpc)	2m 8s
Unit Test Ports (windows-latest, gigadevice/gd32)	1m 0s
Build Examples (stmicro/stm32)	25s
Unit Test Regz (ubuntu-latest)	18s
Unit Test Ports (windows-latest, raspberrypi/rp2xxx)	1m 28s
Build Examples (raspberrypi/rp2xxx)	2m 25s
Unit Test Regz (windows-latest)	1m 22s
Build Examples (wch/ch32v)	2m 14s
Unit Test UF2 (ubuntu-latest)	19s
Unit Test Ports (windows-latest, stmicro/stm32)	1m 34s
Unit Test Regz (macos-latest)	30s
Unit Test UF2 (windows-latest)	1m 13s
Unit Test Ports (macos-latest, gigadevice/gd32)	27s
Unit Test Ports (macos-latest, raspberrypi/rp2xxx)	40s
Unit Test UF2 (macos-latest)	31s
Unit Test Ports (macos-latest, stmicro/stm32)	26s
	34m 44s

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