MAINTAINERS: Add x86 cpuid database entry

Add a specific entry for the x86 architecture cpuid database.

Reference the x86-cpuid.org development mailing list to facilitate easy
tracking by external stakeholders such as the Xen developers.

Include myself as a reviewer.

Note, this MAINTAINERS entry will also cover the auto-generated C cpuid
bitfields header files to be submitted in a future series.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240718134755.378115-10-darwi@linutronix.de