MIPS: alchemy: Fix build error after ioremap cleanup

IOremap changes caused following build error:

arch/mips/alchemy/common/setup.c:99:9: error: implicit declaration of function
+‘remap_pfn_range’; did you mean ‘io_remap_pfn_range’?
+[-Werror=implicit-function-declaration]

Fixed my including linux/mm.h

Fixes: d3991572 ("MIPS: cleanup fixup_bigphys_addr handling")
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/alchemy/common/setup.c

@@ -27,6 +27,7 @@
 
 #include <linux/init.h>
 #include <linux/ioport.h>
+#include <linux/mm.h>
 
 #include <asm/dma-coherence.h>
 #include <asm/mipsregs.h>