include/cpu/msr.h: transform into an union

This makes it easier to get the content of an msr into a full 64bit
variable.

Change-Id: I1b026cd3807fd68d805051a74b3d31fcde1c5626
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68572
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/x86/mtrr/earlymtrr.c

@@ -55,7 +55,7 @@ void set_var_mtrr(
 
 void clear_all_var_mtrr(void)
 {
-	msr_t mtrr = {0, 0};
+	msr_t mtrr = { .raw = 0 };
 	int vcnt;
 	int i;