mpc85xx: Switch to setting the initial stack pointer more clearly

mpc85xx: Switch to setting the initial stack pointer more clearly

Currently, since we know that in the combination of
CONFIG_SYS_INIT_RAM_ADDR + CONFIG_SYS_GBL_DATA_OFFSET all of the "high"
bits are in CONFIG_SYS_INIT_RAM_ADDR and "low" bits are in
CONFIG_SYS_GBL_DATA_OFFSET we reference this separately in start.S, but
added together everywhere else.  For clarity consistency, reference the
combined value here instead.

Signed-off-by: Tom Rini <trini@konsulko.com>