ag7xxx: add initial support for s17

S17 ethernet support is for QCA8337N, which used on
AP152 (QCA9563) board. It is a 7 ports GbE switch.

Signed-off-by: Rosy Song <rosysong@rosinson.com>

Changes for v2-v3:
   - add more commit message for s17

Changes for v4-v5:
   - coding style cleanup
1 file changed