x86/cpuid: Clean up <asm/cpuid/api.h>

 - Include <asm/cpuid/types.h> first, as is customary. This also has
   the side effect of build-testing the header dependency assumptions
   in the types header.

 - No newline necessary after the SPDX line

 - Newline necessary after inline function definitions

 - Rename native_cpuid_reg() to NATIVE_CPUID_REG(): it's a CPP macro,
   whose name we capitalize in such cases.

 - Prettify the CONFIG_PARAVIRT_XXL inclusion block a bit

 - Standardize register references in comments to EAX/EBX/ECX/etc.,
   from the hodgepodge of references.

 - s/cpus/CPUs because why add noise to common acronyms?

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: "Ahmed S. Darwish" <darwi@linutronix.de>
Cc: x86-cpuid@lists.linux.dev
Link: https://lore.kernel.org/r/20250317221824.3738853-4-mingo@kernel.org