[dwuart] Read input clock frequency from the device tree

[dwuart] Read input clock frequency from the device tree

The 16550 design includes a programmable 16-bit clock divider for an
arbitrary input clock, requiring knowledge of the input clock
frequency in order to calculate the divider value for a given baud
rate.  The 16550 UARTs in an x86 PC will always have a 1.8432 MHz
input clock.  Non-x86 systems may have other input clock frequencies.

Define the input clock frequency as a property of a 16550 UART, and
read the value from the device tree "clock-frequency" property.

Signed-off-by: Michael Brown <mcb30@ipxe.org>