fix system config overwrite @ MPC834x and MPC8313

During 83xx setup the "System I/O configuration register high" gets
overwritten with user defined value if CFG_SICRH is defined.

Regarding to the MPC834x manual (Table 5-28 reve.1) bits 28+29 of SICRH
must keep their reset value regardless of configuration.

On my board (using RGMII) those bits are set after reset - yet it's
unclear where they come from.

The patch keeps both bits on MPC834x and MPC8313.

Signed-off-by: Andre Schwarz <andre.schwarz@matrix-vision.de>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>