ddr

This is kind of microseries-within-series indent cleanup.
It is clear that the read_group and write_group variables
have the same value, to just make them into one variable
called rw_group. While doing this, constify the variables
as they are constant.

It is likely that this patch has checkpatch warnings, but
for the sake of not breaking the code, these are ignored.

No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>