arch/unicore32/include/asm/io.h: add readl_relaxed() generic definition

Need generic definition for readl_relaxed(), like other architectures
have done. Or can not pass compiling with allmodconfig, the related
error:

    CC [M]  drivers/message/fusion/mptbase.o
  drivers/message/fusion/mptbase.c: In function 'mpt_send_handshake_request':
  drivers/message/fusion/mptbase.c:1224: error: implicit declaration of function 'readl_relaxed'

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Xuetao Guan <gxt@mprc.pku.edu.cn>