mpc512x: add common LAW and Chip Select configuration

Currently each mpc512x board has its own LAW and CS config code,
we should avoid this code duplication. Allow all boards to use
common code by only defining LAW and CS config macros like
CONFIG_SYS_CSx_START, CONFIG_SYS_CSx_SIZE and CONFIG_SYS_CSx_CFG.

Also allow common configuration of additional CS parameters by
CONFIG_SYS_CS_ALETIMING, CONFIG_SYS_CS_BURST, CONFIG_SYS_CS_DEADCYCLE
and CONFIG_SYS_CS_HOLDCYCLE options.

Signed-off-by: Anatolij Gustschin <agust@denx.de>