serial/efm32-uart: use COMPILE_TEST symbol to extend compile test coverage

The driver fails to build on x86 because writel_relaxed isn't available
there. That function exists on arm, arm64, avr32, hexagon, mips and sh,
but adding all these is overkill so stick to arm only.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -1424,8 +1424,8 @@ config SERIAL_AR933X_NR_UARTS
 	  to support.
 
 config SERIAL_EFM32_UART
-	tristate "EFM32 UART/USART port."
-	depends on ARCH_EFM32
+	tristate "EFM32 UART/USART port"
+	depends on ARM && (ARCH_EFM32 || COMPILE_TEST)
 	select SERIAL_CORE
 	help
 	  This driver support the USART and UART ports on