powerpc: Don't use relative include for config.h in global_data.h

As there is an arch/powerpc/include/asm/config.h file using "" to get
config.h here can lead to using that rather than include/config.h.  This
in turn can lead to a mismatch in the size of gd.

Cc: Matt Merhar <mattmerhar@protonmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Matt Merhar <mattmerhar@protonmail.com>