powerpc/85xx: Implement work-around for P4080 erratum SERDES-A001

Bank powerdown through RCW[SRDS_LPD_Bn] for XAUI on FM2 and SGMII on FM1
are swapped.

Erratum SERDES-A001 says that if bank two is kept disabled and after bank
three is enabled, then the PLL for bank three won't lock properly.  The
work-around is to enable and then disable bank two after bank three is
enabled.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>