The function am33xx_get_cpu_rev may be called before barebox_arm_entry(),
so we need to avoid switch statements. One example is the BeagleBone,
where we use this function to differenciate between the white and black
variants.

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