ARM: mvebu: a38x: move sys_env_device_rev_get

Move sys_env_device_rev_get() from the ddr training code to
sys_env_lib.c (which currently resides with the serdes code). This
brings sys_env_device_rev_get() into line with sys_env_device_id_get()
and sys_env_model_get().

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>