powerpc: Add cpu_secondary_init_r to allow for initialization post env setup

We can simplify some cpu/SoC level initialization by moving it to be
after the environment and non-volatile storage is setup as there might
be dependancies on such things in various boot configurations.

For example for FSL SoC's with QE if we boot from NAND we need it setup
to extra the ucode image to initialize the QE.  If we always do this
after environment & non-volatile storage is working we can have the code
be the same regardless of NOR, NAND, SPI, MMC boot.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>