powerpc/p1010rdb:modify the mtest start_address

In new board P1010RDB-PB, the interrupt vector table is at
the start of memory. So if the start_address needs to be set
a proper value.

Signed-off-by: Zhao Qiang <B45475@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>