powerpc/p1010rdb: update readme for p1010rdb-pa and p1010rdb-pb

- Remove duplicate doc/README.p1010rdb
- Rename README to README.P1010RDB-PA
- Add new README.P1010RDB-PB

P1010RDB-PB is a variation of previous P1010RDB-PA board.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>