bus: vexpress-config: Add explicit of_platform.h include

vexpress-config uses of_platform_* functions which are declared in
of_platform.h. of_platform.h gets implicitly included by of_device.h,
but that is going to be removed soon. Nothing else depends on
of_device.h so it can be dropped.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/r/20230410232727.1562115-1-robh@kernel.org


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>