mb/lenovo/t400/blc.c: Add a new panel to the list

Also adds a comment in the code to clarify what this array is about.

Change-Id: I04b185a5dbd7a7ccb039820f19d2cb549b9a2eac
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/22809
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/mainboard/lenovo/t400/blc.c

@@ -17,6 +17,10 @@
 #include <commonlib/helpers.h>
 #include <northbridge/intel/gm45/gm45.h>
 
+/*
+ * This contains a list of panel IDs and a known well working
+ * backlight PWM frequency.
+ */
 static const struct blc_pwm_t blc_entries[] = {
 	/* Vendor BIOS seems to intend to set PWM freq at 220Hz */
 	/* T400 */
@@ -32,6 +36,7 @@ static const struct blc_pwm_t blc_entries[] = {
 	{"LTN154P2-L05", 110},
 	{"LTN154P3-L02", 110},
 	{"LTN154X3-L02", 110},
+	{"LTN154U2-L05", 110},
 };
 
 int get_blc_values(const struct blc_pwm_t **entries)