powerpc: mpc8548cds: extend the reserved length for monitor

Extend the reserved length for monitor to fix the following
build error:

BINMAN  u-boot-with-dtb.bin
Wrote map file './image.map' to show errors
binman: Section '/binman': contents size 0x80000 (524288)
exceeds section size 0x40000 (262144)
Makefile:1373: recipe for target 'u-boot-with-dtb.bin' failed
make: *** [u-boot-with-dtb.bin] Error 1

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>