rp2040: Add spi slave example #220

Job	Run time
Check that stm32 generated code is up to date	44s
Build (ubuntu-latest)	25s
Build (windows-latest)	1m 48s
Build Website	21s
Build (macos-latest)	41s
Build Examples (espressif/esp)	2m 15s
Build Examples (gigadevice/gd32)	2m 13s
Unit Test Ports (ubuntu-latest, gigadevice/gd32)	18s
Unit Test Regz (ubuntu-latest)	23s
Build Examples (microchip/avr)	2m 19s
Unit Test Regz (windows-latest)	1m 57s
Unit Test Ports (ubuntu-latest, raspberrypi/rp2xxx)	27s
Unit Test UF2 (ubuntu-latest)	22s
Unit Test Regz (macos-latest)	41s
Unit Test Ports (ubuntu-latest, stmicro/stm32)	20s
Unit Test UF2 (windows-latest)	1m 24s
Unit Test UF2 (macos-latest)	26s
Unit Test Ports (windows-latest, gigadevice/gd32)	1m 1s
Build Examples (microchip/atsam)	2m 4s
Unit Test Ports (windows-latest, raspberrypi/rp2xxx)	1m 58s
Build Examples (nordic/nrf5x)	1m 59s
Unit Test Ports (windows-latest, stmicro/stm32)	1m 39s
Dry Run Packaging	1m 22s
Build Examples (nxp/lpc)	2m 3s
Build Examples (stmicro/stm32)	19s
Unit Test Ports (macos-latest, gigadevice/gd32)	44s
Build Examples (raspberrypi/rp2xxx)	2m 26s
Build Examples (wch/ch32v)	2m 29s
Unit Test Ports (macos-latest, raspberrypi/rp2xxx)	45s
Unit Test Ports (macos-latest, stmicro/stm32)	30s
	36m 23s

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