powerpc/powernv: Add interfaces for flash device access

This change adds the OPAL interface definitions to allow Linux to read,
write and erase from system flash devices. We register platform devices
for the flash devices exported by firmware.

We clash with the existing opal_flash_init function, which is really for
the FSP flash update functionality, so we rename that initcall to
opal_flash_update_init().

A future change will add an mtd driver that uses this interface.

Changes from Joel Stanley and Jeremy Kerr.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>