The pixis sgmii command depend on the FPGA support on the board, some 85xx
boards support SGMII riser card but did not support this command, define
CONFIG_PIXIS_SGMII_CMD for those boards which support the sgmii command.

Not like 8544, 8572 has 4 eTsec so that the other two's pixis bits
are not supported by 8544. Therefor, define PIXIS_VSPEED2_MASK and
PIXIS_VCFGEN1_MASK in header file for both boards.

Signed-off-by: Liu Yu <yu.liu@freescale.com>