ls102xa: etsec: Add etsec support for LS102xA

This patch is to add etsec support for LS102xA. First, Little-endian
descriptor mode should be enabled. So RxBDs and TxBDs are interpreted
with little-endian byte ordering. Second, TSEC_SIZE and TSEC_MDIO_OFFSET
are different from PowerPC, redefine them for LS1021xA.

Signed-off-by: Alison Wang <alison.wang@freescale.com>