[MTD] [MAPS] fix platform driver hotplug/coldplug

Since 43cc71eed1250755986da4c0f9898f9a635cb3bf, the platform modalias is
prefixed with "platform:".  Add MODULE_ALIAS() to the hotpluggable MTD mapping
platform drivers, to re-enable auto loading.

NOTE oddness with physmap ...  it's a legacy driver in some configs, which
means it can't always support hotplugging.  (Not that most of these mapping
drivers would often be used as modules...)

[dbrownell@users.sourceforge.net: bugfix, more drivers, registration fixes]
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>