include: configs: ls1028a: set SYS_RX_ETH_BUFFER to 8

LS1028A ethernet interfaces work with at least 8 BDs, set number of buffers
to match that.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>