Since Kernel commit 5b63fb90adb9 ("ARM: dts: Fix incomplete dts data for
am3 and am4 mmc") (barebox commit 419db1f984 ("dts: update to
v5.3-rc7")) the AM33xx MMC2 controller is unconditionally enabled in the
dts. This has the effect that the driver probes for this device and then
can't access the registers as the clock is disabled. Enable the clock to
let the driver probe successfully.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>