unicore32: delete specified xlate_dev_mem_ptr

For the commit <f1ecc698> has changed
xlate_dev_mem_ptr definition in asm-generic/io.h for the systems with
an MMU, so delete it from unicore32 specified io.h.

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jonas Bonn <jonas@southpole.se>