ehea: add kexec support

eHEA resources that are allocated via H_CALLs have a unique identifier each.
These identifiers are necessary to free the resources. A reboot notifier
is used to free all eHEA resources before the indentifiers get lost, i.e
before kexec starts a new kernel.

Signed-off-by: Jan-Bernd Themann <themann@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>