rp2040: Add spi slave example #219

Job	Run time
Unit Test Regz (ubuntu-latest)	17s
Unit Test Regz (windows-latest)	1m 11s
Build Examples (espressif/esp)	2m 7s
Unit Test Regz (macos-latest)	39s
Build Website	26s
Build Examples (gigadevice/gd32)	2m 12s
Build (ubuntu-latest)	25s
Build Examples (microchip/avr)	2m 15s
Build (windows-latest)	1m 14s
Build Examples (microchip/atsam)	1m 59s
Build (macos-latest)	32s
Dry Run Packaging	1m 46s
Build Examples (nordic/nrf5x)	2m 1s
Build Examples (nxp/lpc)	1m 58s
Check that stm32 generated code is up to date	38s
Build Examples (stmicro/stm32)	20s
Unit Test Ports (ubuntu-latest, gigadevice/gd32)	23s
Build Examples (raspberrypi/rp2xxx)	2m 25s
Unit Test Ports (ubuntu-latest, raspberrypi/rp2xxx)	19s
Unit Test UF2 (ubuntu-latest)	22s
Build Examples (wch/ch32v)	2m 21s
Unit Test Ports (ubuntu-latest, stmicro/stm32)	27s
Unit Test UF2 (windows-latest)	1m 0s
Unit Test UF2 (macos-latest)	42s
Unit Test Ports (windows-latest, gigadevice/gd32)	1m 16s
Unit Test Ports (windows-latest, raspberrypi/rp2xxx)	1m 14s
Unit Test Ports (windows-latest, stmicro/stm32)	2m 20s
Unit Test Ports (macos-latest, gigadevice/gd32)	39s
Unit Test Ports (macos-latest, raspberrypi/rp2xxx)	34s
Unit Test Ports (macos-latest, stmicro/stm32)	29s
	34m 31s

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