soc/sifive/fu540: Add helper function to get tlclk frequency

tlclk is not specific to the UART block in the FU540, so let's calculate
its frequency in clock.c.

Change-Id: I270920027f1132253e413a1bf9feb4fe279b651a
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/c/29335
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philipp Hug <philipp@hug.cx>

src/soc/sifive/fu540/clock.c

@@ -250,3 +250,14 @@ int clock_get_coreclk_khz(void)
 		/ (divr + 1)
 		/ (1ul << divq);
 }
+
+/* Get the TileLink clock's frequency, in KHz */
+int clock_get_tlclk_khz(void)
+{
+	/*
+	 * The TileLink bus and most peripherals use tlclk, which is coreclk/2,
+	 * as input.
+	 */
+
+	return clock_get_coreclk_khz() / 2;
+}

src/soc/sifive/fu540/include/soc/clock.h

@@ -18,5 +18,6 @@
 
 void clock_init(void);
 int clock_get_coreclk_khz(void);
+int clock_get_tlclk_khz(void);
 
 #endif /* __SOC_SIFIVE_HIFIFE_U_CLOCK_H__ */

src/soc/sifive/fu540/uart.c

@@ -29,9 +29,5 @@ uintptr_t uart_platform_base(int idx)
 
 unsigned int uart_platform_refclk(void)
 {
-	/*
+	return clock_get_tlclk_khz() * KHz;
-	 * The SiFive UART uses tlclk, which is coreclk/2, as input
-	 */
-
-	return clock_get_coreclk_khz() * KHz / 2;
 }