MX31: drop warnings in get_cpu_rev

Drop warnings due to recent commit
ARM: mx31: Print the silicon version

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>

arch/arm/include/asm/arch-mx31/imx-regs.h

@@ -105,7 +105,7 @@ struct iim_regs {
 
 struct mx3_cpu_type {
 	u8 srev;
-	const char *v;
+	char *v;
 };
 
 #define IOMUX_PADNUM_MASK	0x1ff