xen/arm: Guard correctly asm-arm/platforms/exynos5.h

xen/arm: Guard correctly asm-arm/platforms/exynos5.h

Fix compilation with clang 3.5:

/local/home/julien/works/arndale/xen/xen/include/asm/platforms/exynos5.h:1:9: error: '__ASM_ARM_PLATFORMS_EXYNOS5_H' is used as a header guard here, followed by #define of a different macro [-Werror,-Wheader-guard]
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/local/home/julien/works/arndale/xen/xen/include/asm/platforms/exynos5.h:2:9: note: '__ASM_ASM_PLATFORMS_EXYSNO5_H' is defined here; did you mean '__ASM_ARM_PLATFORMS_EXYNOS5_H'?
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __ASM_ARM_PLATFORMS_EXYNOS5_H

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>