soc/intel/common: Bring DISPLAY_MTRRS into the light

Initially, I wanted to move only the Kconfig DISPLAY_MTRRS into the
"Debug" menu. It turned out, though, that the code looks rather generic.
No need to hide it in soc/intel/.

To not bloat src/Kconfig up any further, start a new `Kconfig.debug`
hierarchy just for debug options.

If somebody wants to review the code if it's 100% generic, we could
even get rid of HAVE_DISPLAY_MTRRS.

Change-Id: Ibd0a64121bd6e4ab5d7fd835f3ac25d3f5011f24
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/29684
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/cpu/x86/mtrr.h

@@ -99,10 +99,17 @@ void x86_mtrr_check(void);
  * This function needs to be called after the first MTRR solution is derived. */
 void mtrr_use_temp_range(uintptr_t begin, size_t size, int type);
 
+static inline int get_var_mtrr_count(void)
+{
+	return rdmsr(MTRR_CAP_MSR).lo & MTRR_CAP_VCNT;
+}
+
 void set_var_mtrr(unsigned int reg, unsigned int base, unsigned int size,
 	unsigned int type);
 int get_free_var_mtrr(void);
 
+asmlinkage void display_mtrrs(void);
+
 /*
  * Set the MTRRs using the data on the stack from setup_stack_and_mtrrs.
  * Return a new top_of_stack value which removes the setup_stack_and_mtrrs data.