p4080ds: add README.p4080ds which documents the "serdes" hwconfig option

Add documentation for the "serdes" hwconfig option, which is used to
specify the status of SerDes banks two and three for the SERDES8 erratum
work-around.

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

doc/README.p4080ds

+Overview
+--------
+The P4080DS is a Freescale reference board that hosts the eight-core P4080 SOC.
+
+SerDes hwconfig configuration
+-----------------------------
+The P4080 RCW includes three sets of bits the specify which SerDes lanes
+should be powered down: SRDS_LPD_B1 (for bank one), SRDS_LPD_B2 (for bank two),
+and SRDS_LPD_B3 (for bank three).  Each of these contains four bits, one for
+each lane in the bank.  SerDes Erratum SERDES8 requires that SRDS_LPD_B2 and
+SRDS_LPD_B3 be set to 0b1111.  This forces banks two and three to be powered
+down at reset.
+
+To re-enable these banks in U-Boot, two hwconfig are available:
+"fsl_srds_lpd_b2" and "fsl_srds_lpd_b3".  The value passed via fsl_srds_lpd_b2
+is written into SRDS_LPD_B2, and the value passed via fsl_srds_lpd_b3 is into
+SRDS_LPD_B3.  Each bit represents one of each bank, and a value of '1'
+indicates that the lane should be powered down.
+
+For example, to indicate that both SerDes banks 2 and 3 are powered down, add
+the following to hwconfig:
+
+	serdes:fsl_srds_lpd_b2=0xf,fsl_srds_lpd_b3=0xf
+
+The "0xf" is a mask that corresponds to the 4 lanes A-D. The most significant
+bit corresponds to lane A.  To indicate that just lane A of bank 3 is to be
+powered down, use:
+
+	serdes:fsl_srds_lpd_b3=8
+
+These options should be specified only if U-Boot does not automatically power
+on the correct lanes.