soc/amd/stoneyridge: use SMBus speed in compilation unit

The fixed bus speed of 400 kHz doesn't need to reside in a header file.
Just move the assumption into the code itself.

Change-Id: I426fe078909a9b725c1747380d69af31292b6d1e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38611
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/soc/amd/stoneyridge/southbridge.c b/src/soc/amd/stoneyridge/southbridge.c
index 1b2afec..b0aaf24 100644
--- a/src/soc/amd/stoneyridge/southbridge.c
+++ b/src/soc/amd/stoneyridge/southbridge.c
@@ -378,8 +378,11 @@
 
 static void fch_smbus_init(void)
 {
+	/* 400 kHz smbus speed. */
+	const uint8_t smbus_speed = (66000000 / (400000 * 4));
+
 	pm_write8(SMB_ASF_IO_BASE, SMB_BASE_ADDR >> 8);
-	smbus_write8(SMBTIMING, SMB_SPEED_400KHZ);
+	smbus_write8(SMBTIMING, smbus_speed);
 	/* Clear all SMBUS status bits */
 	smbus_write8(SMBHSTSTAT, SMBHST_STAT_CLEAR);
 	smbus_write8(SMBSLVSTAT, SMBSLV_STAT_CLEAR);