powerpc/P1010RDB:Update RESET_VECTOR_ADDRESS for 768KB u-boot size

U-boot binary size has been increased from 512KB to 768KB.

So update CONFIG_RESET_VECTOR_ADDRESS to reflect the same for
P1010 SPI Flash Secure boot target.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
[York Sun: Modified subject to P1010RDB]
Reviewed-by: York Sun <yorksun@freescale.com>