mips: ath79: Fix compiler warning on const assignment

The assignment const T var; var = value; is illegal, since var is
constant. Drop the const to fix the compiler warning.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>

arch/mips/mach-ath79/reset.c

@@ -46,7 +46,7 @@ void _machine_restart(void)
 
 u32 get_bootstrap(void)
 {
-	const void __iomem *base;
+	void __iomem *base;
 	u32 reg = 0;
 
 	base = map_physmem(AR71XX_RESET_BASE, AR71XX_RESET_SIZE,