powerpc/powernv: Invoke opal_cec_reboot2() on unrecoverable HMI.

Invoke new opal_cec_reboot2() call with reboot type
OPAL_REBOOT_PLATFORM_ERROR (for unrecoverable HMI interrupts) to inform
BMC/OCC about this error, so that BMC can collect relevant data for error
analysis and decide what component to de-configure before rebooting.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>