src/cpu/x86/mtrr/earlymtrr: Add clear_all_var_mtrr

Picasso does not define the state of variable MTRRs on boot. Add a
helper function to clear all MTRRs.

BUG=b:147042464
TEST=Build trembyle

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I21b887ce12849a95ddd8f1698028fb6bbfb4a7f6
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40764
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/x86/mtrr.h

@@ -107,6 +107,7 @@ static inline int get_var_mtrr_count(void)
 void set_var_mtrr(unsigned int reg, unsigned int base, unsigned int size,
 	unsigned int type);
 int get_free_var_mtrr(void);
+void clear_all_var_mtrr(void);
 
 asmlinkage void display_mtrrs(void);