platform: Remove most references to platform_bus device

A number of board files in arch/arm and arch/unicore32
explicitly reference platform_bus device as a parent
for new platform devices.

This is unnecessary, as platform device API guarantees
that devices with NULL parent are going to by adopted
by the mentioned "root" device.

This patch removes or replaces with NULL such references.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>