x86/of: Change x86_dtb_parse_smp_config() to static

x86_dtb_parse_smp_config() is called locally only, change it to static.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/1712068830-4513-5-git-send-email-ssengar@linux.microsoft.com