mb/lenovo/t400: Add 154WX5-TLB2 to display backlight PWM freq list

Vendor VBT on ThinkPad R500 intends to set a 220MHz backlight PWM frequency
which seems to work well.

Change-Id: Ic1a12c7e3173468561ed5615319962d0abc6d61b
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/28643
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/lenovo/t400/blc.c

@@ -37,6 +37,8 @@ static const struct blc_pwm_t blc_entries[] = {
 	{"LTN154P3-L02", 110},
 	{"LTN154X3-L02", 110},
 	{"LTN154U2-L05", 110},
+	/* R500 */
+	{"154WX5-TLB2", 220},
 };
 
 int get_blc_values(const struct blc_pwm_t **entries)