drivers/uart: Enable override for input clock divider

Allow the platform to override the input clock divider by adding the
uart_input_clock_divider routine.  This routine combines the baud-rate
oversample divider with any other input clock divider.  The default
routine returns 16 which is the standard baud-rate oversampling value.
A platform may override this default "weak" routine by providing a new
routine and selecting UART_OVERRIDE_INPUT_CLOCK_DIVIDER.  This works
around ROMCC not supporting weak routines.

Testing on Galileo:
*  Edit the src/mainboard/intel/galileo/Makefile.inc file:
   *  Add "select ADD_FSP_PDAT_FILE"
   *  Add "select ADD_FSP_RAW_BIN"
   *  Add "select ADD_RMU_FILE"
*  Place the FSP.bin file in the location specified by CONFIG_FSP_FILE
*  Place the pdat.bin files in the location specified by
   CONFIG_FSP_PDAT_FILE
*  Place the rmu.bin file in the location specified by CONFIG_RMU_FILE
*  Build EDK2 CorebootPayloadPkg/CorebootPayloadPkgIa32.dsc to generate
   UEFIPAYLOAD.fd
*  Testing is successful when CorebootPayloadPkg is able to properly
   initialize the serial port without using built-in values.

Change-Id: Ieb6453b045d84702b8f730988d0fed9f253f63e2
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/14611
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/console/uart.h

@@ -35,6 +35,10 @@ unsigned int default_baudrate(void);
 unsigned int uart_baudrate_divisor(unsigned int baudrate,
 	unsigned int refclk, unsigned int oversample);
 
+/* Returns the oversample divisor multiplied by any other divisors that act
+ * on the input clock
+ */
+unsigned int uart_input_clock_divider(void);
 
 void uart_init(int idx);
 void uart_tx_byte(int idx, unsigned char data);