ppc4xx: Enable booting with Option E on 460EX/EXr/GT

This patch enables booting with option E on the PPC460EX/EXr/GT.
When booting with Option E, the PLL is in bypass, CPR0_PLLC[ENG]=0.
The Software Boot Configuration Procedure is needed to engage the
PLL and perform a chip reset.

Signed-off-by: Stefan Roese <sr@denx.de>