powerpc: Replace CONFIG_SYS_INIT_RAM_END with CONFIG_SYS_INIT_RAM_SIZE

powerpc: Replace CONFIG_SYS_INIT_RAM_END with CONFIG_SYS_INIT_RAM_SIZE

CONFIG_SYS_INIT_RAM_SIZE may be used out of the board header file.
Some boards use CONFIG_SYS_INIT_RAM_END for the same purpose. To
unify the macros, use CONFIG_SYS_INIT_RAM_SIZE for all.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Mario Six <mario.six@gdsys.cc>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>