platform/x86: p2sb: Make the Kconfig symbol hidden

platform/x86: p2sb: Make the Kconfig symbol hidden

The P2SB is used purely as a library and all users must select it with

	depends on PCI
	select P2SB if X86

statement. Without this the combination of different configuration
options may lead to build failures.

Suggested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230822142514.2140897-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>