ARM: mach-shmobile: Flush caches in platform_cpu_die()

Add cache flushing code to the SH-Mobile specific CPU hotplug
implementation. While at it, add a cpu mask to make sure the
cache flushing code is finished in platform_cpu_die() before
letting the SoC-specific code in shmobile_platform_cpu_kill()
proceed with turning off power.

Without this code CPU hotplug offline fails when cache is
enabled on Cortex-A9 based SoCs.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>