[V2 PATCH] socfpga_a10: fix a kexec boot issue

[<yanjiang.jin@windriver.com>]

From: Yanjiang Jin <yanjiang.jin@windriver.com>


V1->V2:
1. Add flush_cache_all() in cpu_die();
2. Add more descriptive on the commit log;
Also validate kexec and CPU_HOTPLUG as before.

I guess socfpga's other boards may need this patch, but I have Arria10
board only. 
So add a new function socfpga_a10_cpu_kill(), keep the old function
socfpga_cpu_kill() for other boards.
I also verified CPU_HOTPLUG after applying this patch, everything seems well.

Test steps:

1. Enable kexec and build a SOCFPGA kernel;
2. Use zImage as 1st and 2nd kernel;
3. kexec -l /root/zImage --append="`cat /proc/cmdline`" 
4. kexec -e

Test env:

U-Boot 2014.10 (Jan 13 2016 - 11:07:09)

CPU   : Altera SOCFPGA Arria 10 Platform
BOARD : Altera SOCFPGA Arria 10 Dev Kit

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1.9.1