microblaze: consolidate GPIO reset handling

Now that platform.c only has the GPIO reset handling left, move the
initcall to reset.c and remove platform.c.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>