ram: k3-ddrss: do not touch ctrl regs during training

During LPDDR initialization we will loop through a series of frequency
changes in order to train at the various operating frequencies. During
this training, accessing the DRAM_CLASS bitfield could happen during a
frequency change and cause the read to hang.

Store the DRAM type into the main structure to avoid multiple readings
while the independent phy is training.

Signed-off-by: Bryan Brattlof <bb@ti.com>