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>