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>