configs: arbel: increase u-boot mapping size

When u-boot enable CONFIG_SYS_BOOT_RAMDISK_HIGH, rootfs image relocated
from FIU address space to memory address before jump to kernel.

Since Arbel reserved memory from 0x00000000 to 0x06200000 for tip image,
and rootfs image may too large that cannot found a suitable location
before 128MB(0x8000000), so increase mapping size from 128MB to 192MB.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>