[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>

src/arch/x86/core/x86_uart.c

@@ -37,6 +37,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ISA_UART( NAME, BASE )						\
 	static struct ns16550_uart ns16550_ ## NAME = {			\
 		.base = ( ( void * ) (BASE) ),				\
+		.clock = NS16550_CLK_DEFAULT,				\
 	};								\
 	struct uart NAME = {						\
 		.refcnt = REF_INIT ( ref_no_free ),			\