[ARM] S3C: Update UART save over PM suspend/resume

Change the way the UART state is saved over suspend to allow the s3c64xx
code to modify the settings on resume to avoid any illegal state changes
to the UART clocks. This will also allow us to save the UDIVSLOT register
on newer SoCs.

Move to using a structure for the UART use the extant Kconfig configuration
specifying the number of UARTs.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>