realtek: pcs: add more SerDes access helpers

realtek: pcs: add more SerDes access helpers

Add more SerDes access helpers for the upcoming code import from PHY
driver. There, similar helpers are used to read and write full SerDes
registers or only parts of them (aka bitfields).

The helpers are expected to replace the following used in PHY SerDes
code:
  - rtl9300_sds_field_r
  - rtl9300_sds_field_w
  - rtsds_931x_read
  - rtsds_931x_read_field
  - rtsds_931x_write
  - rtsds_931x_write_field

Mark the helpers as unused for now to make the compiler happy. This will
be removed as soon as they are used.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20352
Signed-off-by: Robert Marko <[email protected]>