ravb: remove APSR_DM

According to the R-Car Series, 3rd Generation User's Manual: Hardware,
Rev. 1.50, there's no APSR.DM field, instead there are 2 independent
RX/TX clock internal delay bits. Follow the suit: remove #define APSR_DM
and rename #define's APSR_DM_{R|T}DM to APSR_{R|T}DM.

While at it, do several more things to the declaration of *enum* APSR_BIT:
- remove superfluous indentation;
- annotate APSR_MEMS as undocumented;
- annotate APSR as R-Car Gen3 only.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omprussia.ru>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>