PowerPC: Stop re-using CONFIG_SPL_TEXT_BASE for TPL

Rather than checking for CONFIG_TPL_BUILD and then re-defining
CONFIG_SPL_TEXT_BASE make use of CONFIG_TPL_TEXT_BASE directly.

Cc: York Sun <york.sun@nxp.com>
Cc: Po Liu <po.liu@nxp.com>
Cc: Qiang Zhao <qiang.zhao@nxp.com>
Cc: Timur Tabi <timur@tabi.org>
Signed-off-by: Tom Rini <trini@konsulko.com>