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

arch/unicore32/include/asm/io.h

@@ -39,6 +39,10 @@ extern void __uc32_iounmap(volatile void __iomem *addr);
 #define ioremap_nocache(cookie, size)	__uc32_ioremap(cookie, size)
 #define iounmap(cookie)			__uc32_iounmap(cookie)
 
+#define readb_relaxed readb
+#define readw_relaxed readw
+#define readl_relaxed readl
+
 #define HAVE_ARCH_PIO_SIZE
 #define PIO_OFFSET		(unsigned int)(PCI_IOBASE)
 #define PIO_MASK		(unsigned int)(IO_SPACE_LIMIT)