[POWERPC] Abolish powerpc_flash_init()

powerpc_flash_init() implements a broken way of probing for flash
devices supported by the physmap_of driver.  It finds all nodes in the
device tree with device_type=="rom" and instantiates of_platform
devices for them.  This is fundamentally incompatible with the normal
and correct way of probing for of_platform_bus_probe().  Platforms
which relied on powerpc_flash_init()s behaviour (none are in-tree)
will have to update their platform probing code to correctly probe
busses containing flash devices.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>