MIPS: ralink: Fix build error due to missing header

Previously, <linux/module.h> was included before ralink_regs.h in all
ralink files - leading to <linux/io.h> being implicitly included.

After commit 26dd3e4 ("MIPS: Audit and remove any unnecessary
uses of module.h") removed the inclusion of module.h from multiple
places, some ralink platforms failed to build with the following error:

In file included from arch/mips/ralink/mt7620.c:17:0:
./arch/mips/include/asm/mach-ralink/ralink_regs.h: In function ‘rt_sysc_w32’:
./arch/mips/include/asm/mach-ralink/ralink_regs.h:38:2: error: implicit declaration of function ‘__raw_writel’ [-Werror=implicit-function-declaration]
  __raw_writel(val, rt_sysc_membase + reg);
  ^
./arch/mips/include/asm/mach-ralink/ralink_regs.h: In function ‘rt_sysc_r32’:
./arch/mips/include/asm/mach-ralink/ralink_regs.h:43:2: error: implicit declaration of function ‘__raw_readl’ [-Werror=implicit-function-declaration]
  return __raw_readl(rt_sysc_membase + reg);

Fix this by including <linux/io.h>.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Fixes: 26dd3e4 ("MIPS: Audit and remove any unnecessary uses of module.h")
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> #4.11+
Patchwork: https://patchwork.linux-mips.org/patch/16780/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mach-ralink/ralink_regs.h

@@ -13,6 +13,8 @@
 #ifndef _RALINK_REGS_H_
 #define _RALINK_REGS_H_
 
+#include <linux/io.h>
+
 enum ralink_soc_type {
 	RALINK_UNKNOWN = 0,
 	RT2880_SOC,