x86: p2sb: make P2SB driver depend on P2SB uclass

Currently it is possible to select the P2SB driver without selecting the
P2SB uclass, which can't work. Fix this by adding a "depends on" in
Kconfig.
Signed-off-by: NWolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>

arch/x86/Kconfig

@@ -717,6 +717,7 @@ config HAVE_ITSS
 
 config HAVE_P2SB
 	bool "Enable P2SB"
+	depends on P2SB
 	help
 	  Select this to include the driver for the Primary to
 	  Sideband Bridge (P2SB) which is found on several Intel