ARM: OMAP2/3: Serial: Remove arch_initcall dependency

Move platform_device_register() for serial device to
omap_serial_init()

There is no need to have arch_initcall() dependency in serial
as already board files call the function omap_serial_init()

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>