net/fm: fix MDIO controller base on FMAN2

MDIO controller base on FMAN2 was defined as CONFIG_SYS_FSL_FM2_ADDR
plus offset, but CONFIG_SYS_FSL_FM2_ADDR only defined when there are two
FMANs, so we should only define MDIO controller base on FMAN2 when there
is FMAN2.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Mingkai Hu <Mingkai.Hu@freescale.com>
Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>