PowerPC: Rework PBL location choice

When converting CONFIG_SDCARD and CONFIG_SPIFLASH to Kconfig, one set of
uses wasn't converted correctly. Allow for the case where platforms
don't rely on "PBL" to boot but instead use other mechanisms. See the
link below for more details.

Link: https://lore.kernel.org/all/20220802091338.f4g45ldhc7qbg6hm@pali/


Fixes: d433c74e ("Convert CONFIG_SDCARD et al to Kconfig")
Tested-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>