ARM: ux500: Fix build error due to missing include of asm/pmu.h in cpu-db8500.c

ARM: ux500: Fix build error due to missing include of asm/pmu.h in cpu-db8500.c

Include asm/pmu.h to fix below build error:

  CC      arch/arm/mach-ux500/cpu-db8500.o
arch/arm/mach-ux500/cpu-db8500.c:118:8: error: variable 'db8500_pmu_platdata' has initializer but incomplete type
arch/arm/mach-ux500/cpu-db8500.c:119:2: error: unknown field 'handle_irq' specified in initializer
arch/arm/mach-ux500/cpu-db8500.c:119:2: warning: excess elements in struct initializer [enabled by default]
arch/arm/mach-ux500/cpu-db8500.c:119:2: warning: (near initialization for 'db8500_pmu_platdata') [enabled by default]
make[1]: *** [arch/arm/mach-ux500/cpu-db8500.o] Error 1
make: *** [arch/arm/mach-ux500] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>