powerpc: Switch to using CONFIG_SYS_INIT_SP_OFFSET from CONFIG_SYS_GBL_DATA_OFFSET

powerpc: Switch to using CONFIG_SYS_INIT_SP_OFFSET from CONFIG_SYS_GBL_DATA_OFFSET

In the places where PowerPC references CONFIG_SYS_GBL_DATA_OFFSET it
does so as (CONFIG_SYS_INIT_RAM_ADDR + CONFIG_SYS_GBL_DATA_OFFSET).  And
it defines CONFIG_SYS_GBL_DATA_OFFSET in the same manner that other
architectures define CONFIG_SYS_INIT_SP_OFFSET. Other architectures
define CONFIG_SYS_INIT_SP_ADDR as (CONFIG_SYS_INIT_RAM_ADDR +
CONFIG_SYS_INIT_SP_OFFSET) typically.  Rename things within PowerPC for
consistency with other architectures.

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