clk: Drop unused efm32gg driver

Support for this machine was just removed, so drop the now unused clk
driver, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210114151630.128830-3-u.kleine-koenig@pengutronix.de


Signed-off-by: Stephen Boyd <sboyd@kernel.org>