USB: s3c-hsotg: Add support for external USB clock

The PLL that drives the USB clock supports 3 input clocks: 12, 24 and 48Mhz.
This patch adds support to the USB driver for setting the correct register bit
according to the given clock.

This depends on the following patch:
[PATCH] ARM: S3C64XX: Add USB external clock definition

Signed-off-by: Maurus Cuelenaere <mcuelenaere@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>