ppc4xx: Fix PCIE PLL lock on 440SPe Yucca board

u-boot reports a PCIE PLL lock error at boot time on Yucca board, and
left PCIe nonfunctional. This is fixed by making u-boot function
ppc4xx_init_pcie() to wait 300 uS after negating reset before the
first check of the PLL lock.

Signed-off-by: Rupjyoti Sarmah <rsarmah@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>