x86/dtb: Rename x86_dtb_init()

x86_dtb_init() is a misnomer and it really should be used as a SMP
configuration parser which is selected by the platform via
x86_init::mpparse:parse_smp_config().

Rename it to x86_dtb_parse_smp_config() in preparation for that.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20240212154639.495992801@linutronix.de