of: Add vendor prefix for Energy Micro

The kernel supports devices with the following compatible strings
already:

	energymicro,efm32-i2c
	energymicro,efm32-uart
	energymicro,efm32-spi
	energymicro,efm32-timer

So add "energymicro" to the list of vendors.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>