ARM: EXYNOS: Use IOMEM_ERR_PTR when function returns iomem

Function returns 'void __iomem *' so use IOMEM_ERR_PTR for returning
an error. This fixes sparse warning:

arch/arm/mach-exynos/platsmp.c:185:31: warning: incorrect type in return expression (different address spaces)
arch/arm/mach-exynos/platsmp.c:185:31:    expected void [noderef] <asn:2>*
arch/arm/mach-exynos/platsmp.c:185:31:    got void *

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>

arch/arm/mach-exynos/platsmp.c

@@ -182,7 +182,7 @@ static inline void __iomem *cpu_boot_reg(int cpu)
 
 	boot_reg = cpu_boot_reg_base();
 	if (!boot_reg)
-		return ERR_PTR(-ENODEV);
+		return IOMEM_ERR_PTR(-ENODEV);
 	if (soc_is_exynos4412())
 		boot_reg += 4*cpu;
 	else if (soc_is_exynos5420() || soc_is_exynos5800())