mb/lenovo/x200/blc: Add LTN121AT07-L02 at 750Hz

Its EDID string is "LTN121AT07L02". The vendor sets BLC_PWM_CTL to
0x31313131.

This frequency seems working well on the x200 with this panel, which
is said to be LED.

Change-Id: I8b0ec04c6f6fcb6d4027a5114698db87d7718191
Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74182
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/mainboard/lenovo/x200/blc.c

@@ -18,6 +18,7 @@ static const struct blc_pwm_t blc_entries[] = {
 	{"B121EW09 V3 ", 400},
 	{"HV121WX4-120", 110}, /* Aftermarket AFFS lcd, works well at low pwm */
 	{"LTN121AT03001", 110},
+	{"LTN121AT07L02", 750}, /* Its part name is LTN121AT07-L02 */
 	{"LTN121AP03001", 750},
 	/* TODO: untested panels found on thinkwiki */
 	/* Generally CCFL runs best at lower PWM */