spi: omap2: Add build dependencies for writel_relaxed()

It's not reliably available.
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/Kconfig

@@ -301,6 +301,7 @@ config SPI_OMAP_UWIRE
 
 config SPI_OMAP24XX
 	tristate "McSPI driver for OMAP"
+	depends on ARM || ARM64 || AVR32 || HEXAGON || MIPS || SH
 	depends on ARCH_OMAP2PLUS || COMPILE_TEST
 	help
 	  SPI master controller for OMAP24XX and later Multichannel SPI