nb/intel/haswell: Implement proper backlight PWM config

Further backport the backlight-PWM handling from Skylake. Beside
configuring the PWM frequency in Hz, we also use the PCH's logic
for the brightness setting via BLM_PCH_OVERRIDE_ENABLE. Linux
would toggle it anyway and that might confuse our ASL code.

We assume that the 183Hz value that was set before for Slippy
variants was overridden by Linux with the 200Hz VBT value, like
it was for the Broadwell Chromebooks. So we set 200Hz for them
in the devicetrees. The calculated value for the T440p of 220Hz
seems sane and also matches the VBT.

Change-Id: I17dfe1a3610d5e2918c617cf5d10896692fdccb3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39769
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/drivers/intel/gma/i915_reg.h

@@ -3580,6 +3580,7 @@
 #define SOUTH_CHICKEN2		0xc2004
 #define  FDI_MPHY_IOSFSB_RESET_STATUS	(1<<13)
 #define  FDI_MPHY_IOSFSB_RESET_CTL	(1<<12)
+#define  LPT_PWM_GRANULARITY		(1<<5)
 #define  DPLS_EDP_PPS_FIX_DIS		(1<<0)
 
 #define _FDI_RXA_CHICKEN         0xc200c