x86: fix: make PCI ECS for AMD CPUs hotplug capable

Until now, PCI ECS setup was performed at boot time only and for cpus
that are enabled then. This patch fixes this and adds cpu hotplug.

Tests sequence (check if ECS bit is set when bringing cpu online again):

 # ( perl -e 'sysseek(STDIN, 0xC001001F, 0)'; hexdump -n 8 -e '2/4 "%08x " "\n"' )   < /dev/cpu/1/msr
 00000008 00404010
 # ( perl -e 'sysseek(STDOUT, 0xC001001F, 0); print pack "l*", 8, 0x00400010' ) > /dev/cpu/1/msr
 # ( perl -e 'sysseek(STDIN, 0xC001001F, 0)'; hexdump -n 8 -e '2/4 "%08x " "\n"' )   < /dev/cpu/1/msr
 00000008 00400010
 # echo 0 > /sys/devices/system/cpu/cpu1/online
 # echo 1 > /sys/devices/system/cpu/cpu1/online
 # ( perl -e 'sysseek(STDIN, 0xC001001F, 0)'; hexdump -n 8 -e '2/4 "%08x " "\n"' )   < /dev/cpu/1/msr
 00000008 00404010

Reported-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>