serial: arc_uart: Fix module alias

Platform drivers use "platform:" prefix in module alias.
Also use DRIVER_NAME in MODULE_ALIAS to make module autoloading work.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>