ARM: efm32: switch to vendor,device compatible strings

The mainline drivers were fixed in commits

 - 4ea8dafd (serial: efm32: use $vendor,$device scheme for
   compatible string)
 - 12f6dd86 (spi: efm32: use $vendor,$device scheme for compatible
   string)
 - 63cc1223 (clocksource: efm32: use $vendor,$device scheme for
   compatible string)

which are all in 3.16-rc1 to also support these compatible strings.

For the other affected peripherals (adc and gpio) there are no drivers in
mainline yet.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>