nb/gm45/gma.c: Compute BLC_PWM_CTL value from PWM frequency

This allows to set the backlight PWM frequency and the
duty cycle in the devicetree instead of using a plain BLC_PWM_CTL
value.

Change-Id: I4d9a555ac7ea5605712c1fcda994a6fcabf9acf3
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/17597
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/src/northbridge/intel/gm45/chip.h b/src/northbridge/intel/gm45/chip.h
index 836d6bb..a281ee9 100644
--- a/src/northbridge/intel/gm45/chip.h
+++ b/src/northbridge/intel/gm45/chip.h
@@ -26,6 +26,8 @@
 	u16 gpu_panel_power_backlight_off_delay; /* Tx time sequence */
 	u8 gpu_panel_power_cycle_delay;          /* T4 time sequence */
 	struct i915_gpu_controller_info gfx;
+	u16 pwm_freq;
+	u8 duty_cycle;
 
 	/*
 	 * Maximum PCI mmio size in MiB.