ARM: EXYNOS: use u8 for val[] in struct exynos_pmu_conf

Values stored in val[] are never bigger than a byte.

   text    data     bss     dec     hex filename
   7716    3692       8   11416    2c98 arch/arm/mach-exynos/pmu.o.before
   5436    1908       8    7352    1cb8 arch/arm/mach-exynos/pmu.o.after

Cc: Pankaj Dubey <pankaj.dubey@samsung.com>
Cc: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>