x86/microcode/AMD: Add get_patch_level()

Put the MSR_AMD64_PATCH_LEVEL reading of the current microcode revision
the hw has, into a separate function.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20250211163648.30531-6-bp@kernel.org