mb/emulation/qemu-q35: don't select CONFIG_UNKNOWN_TSC_RATE

This option signifies missing implementation of tsc_freq_mhz() which
results in tsc_freq_mhz() from src/arch/x86/timestamp.c returning zero.
That zero in turn gets put into TIMESTAMP CBMEM table requiring payloads
and user-space tools to figure out the frequency on their own.

Implement tsc_freq_mhz() to query timer frequency in coreboot, so it
gets reported downstream.

Change-Id: I88d1206c13f15a9f20c07b65dcec42ec614f7e6a
Upstream-Status: Pending
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>

src/cpu/qemu-x86/Kconfig

@@ -6,7 +6,6 @@ config CPU_QEMU_X86
 	select HAVE_X86_64_SUPPORT
 	select UDELAY_TSC
 	select TSC_MONOTONIC_TIMER
-	select UNKNOWN_TSC_RATE
 	select NEED_SMALL_2MB_PAGE_TABLES	# QEMU doesn't support 1GB pages
 	select IDT_IN_EVERY_STAGE
 

src/mainboard/emulation/qemu-q35/Makefile.mk

@@ -18,9 +18,11 @@ ramstage-y += cpu.c
 
 all-y += ../qemu-i440fx/fw_cfg.c
 all-y += ../qemu-i440fx/bootmode.c
+all-y += timer.c
 
 ramstage-$(CONFIG_CHROMEOS) += chromeos.c
 
 smm-y += ../qemu-i440fx/rom_media.c
 smm-y += memmap.c
 smm-y += smihandler.c
+smm-y += timer.c

src/mainboard/emulation/qemu-q35/timer.c

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <cpu/x86/tsc.h>
+
+unsigned long tsc_freq_mhz(void)
+{
+	/* ACPI's Power Management Timer frequency is fixed at 3.579545 MHz. */
+	return 3579545 / 1000000;
+}