soc/sifive/fu540: Simplify UART refclk calculation

clock_get_coreclk_khz() already detects whether the PLL or the input
clock (hfclk) is used.

Tested on HiFive Unleashed.

Change-Id: I264977b0de0b81ef74a014984b6d33638ab33f4b
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/c/29334
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Hug <philipp@hug.cx>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/soc/sifive/fu540/Makefile.inc

@@ -17,6 +17,7 @@ bootblock-y += uart.c
 bootblock-y += clint.c
 bootblock-y += media.c
 bootblock-y += bootblock.c
+bootblock-y += clock.c
 
 romstage-y += uart.c
 romstage-y += clint.c

src/soc/sifive/fu540/clock.c

@@ -242,6 +242,7 @@ void clock_init(void)
 }
 #endif /* ENV_ROMSTAGE */
 
+/* Get the core clock's frequency, in KHz */
 int clock_get_coreclk_khz(void)
 {
 	if (read32(&prci->coreclksel) & PRCI_CORECLK_MASK)

src/soc/sifive/fu540/uart.c

@@ -30,11 +30,8 @@ uintptr_t uart_platform_base(int idx)
 unsigned int uart_platform_refclk(void)
 {
 	/*
-	 * The SiFive UART uses tlclk, which is coreclk/2 as input
+	 * The SiFive UART uses tlclk, which is coreclk/2, as input
 	 */
 
-	if (ENV_BOOTBLOCK)
-		return 33330000 / 2;
-	else
 	return clock_get_coreclk_khz() * KHz / 2;
 }