clk: new driver for efm32 SoC

This patch adds support for the clocks provided by the Clock Management
Unit of Energy Micro's efm32 Giant Gecko SoCs including device tree
bindings.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>