powerpc/powernv: Rename machine_check_pSeries_early() to powernv

The routine machine_check_pSeries_early() is only used on powernv, not
pseries. Hence rename machine_check_pSeries_early() to
machine_check_powernv_early().

Reported-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>