greybus: arche_platform: Remove child's platform device as part of _remove() fn

It seems we need to delete platform_dev of all childs explicitly,
in _remove() fn callback of parent driver.
There were some discussions about having of_platform_unpopulate(),
but it never made it to mainline.

https://lkml.org/lkml/2013/7/19/615



There are some drivers which are removing platform_dev explicitly,
as done in this patch.

Note that, without this, multiple insmod-rmmod won't work, as I see
driver probe gets invoked twice and leads to failure of probe.

Signed-off-by: Vaibhav Hiremath <hiremath_vaibhav@projectara.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>