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>

arch/microblaze/kernel/Makefile

@@ -16,7 +16,7 @@ extra-y := head.o vmlinux.lds
 
 obj-y += dma.o exceptions.o \
 	hw_exception_handler.o irq.o \
-	platform.o process.o prom.o ptrace.o \
+	process.o prom.o ptrace.o \
 	reset.o setup.o signal.o sys_microblaze.o timer.o traps.o unwind.o
 
 obj-y += cpu/