powerpc/85xx: Fix build failure for P1023RDS

powerpc/85xx: Fix build failure for P1023RDS

When we added the fman fdt fixup we forgot to fixup the P1023RDS
platform.  So we would get:

fdt.c: In function 'fdt_fixup_fman_firmware':
fdt.c:465:15: error: 'CONFIG_SYS_FMAN_FW_LENGTH' undeclared (first use in this function)
fdt.c:465:15: note: each undeclared identifier is reported only once for each function it appears in

Add the needed #defines in P1023RDS.h to fix this.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>