powerpc/85xx: introduce and document CONFIG_SYS_CCSRBAR macros

Introduce the CONFIG_SYS_CCSRBAR_PHYS_HIGH and CONFIG_SYS_CCSRBAR_PHYS_LOW
macros, which contain the high and low portions of CONFIG_SYS_CCSRBAR_PHYS.
This is necessary for the assembly-language code that relocates CCSR, since
the assembler does not understand 64-bit constants.

CONFIG_SYS_CCSRBAR_PHYS is automatically defined from the
CONFIG_SYS_CCSRBAR_PHYS_HIGH and CONFIG_SYS_CCSRBAR_PHYS_LOW macros, so it
should not be defined in a board header file.  Similarly,
CONFIG_SYS_CCSRBAR_DEFAULT is defined for each SOC in config_mpc85xx.h, so
it should also not be defined in the board header file.

CONFIG_SYS_CCSR_DO_NOT_RELOCATE is a "short-cut" macro that guarantees that
CONFIG_SYS_CCSRBAR_PHYS is set to the same value as CONFIG_SYS_CCSRBAR_DEFAULT,
and so CCSR will not be relocated.

Since CONFIG_SYS_CCSRBAR_DEFAULT is locked to a fixed value, multi-stage U-Boot
builds (e.g. NAND) are required to relocate CCSR only during the last stage
(i.e. the "real" U-Boot).  All other stages should define
CONFIG_SYS_CCSR_DO_NOT_RELOCATE to ensure that CCSR is not relocated.

README is updated with descriptions of all the CONFIG_SYS_CCSRBAR_xxx macros.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>