unicore32: change zImage physical address, though it's PIC codes

U-boot will load the kernel image to 48M physical memory address.
The patch changes it to the correct address, though it's PIC codes.

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Acked-by: Arnd Bergmann <arnd@arndb.de>