mpc85xx: Base emulator support

Prepare for emulator support for mpc85xx parts.
Disable DDR training and skip wrlvl_cntl_2 and wrlvl_cntl_3 registers.
These two registers improve stability but not supported by emulator.
Add CONFIG_FSL_TBCLK_EXTRA_DIV for possible adjustment to time base.

Signed-off-by: York Sun <yorksun@freescale.com>