of/platform: Register of_platform_drivers with an "of:" prefix

Currently there are some drivers in tree which register both a
platform_driver and an of_platform_driver with the same name.  This is
a temporary situation until all the relevant of_platform_drivers are
converted to be normal platform_drivers.  Until then, this patch gives
all the of_platform_drivers an "of:" prefix to protect against bogus
matches and namespace conflicts.