Make setting MAX_PIRQ_LINKs depend on NORTHBRIDGE_VIA_VX900

The MAX_PIRQ_LINKS is defined in src/Kconfig with a default value
of 4. The src/northbridge/via/vx900/Kconfig also defines
MAX_PIRQ_LINKS with a default of 8 and it ends up giving us
a value of 8 for non-VIA platforms.

Change-Id: Iee1938d38a93ab7c35c8cb6fe9656a92cf3fa21e
Signed-off-by: Dave Frodin <dave.frodin@se-eng.com>
Reviewed-on: http://review.coreboot.org/3586
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.huber@secunet.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/northbridge/via/vx900/Kconfig

@@ -28,6 +28,8 @@ config NORTHBRIDGE_VIA_VX900
 	select MMCONF_SUPPORT_DEFAULT
 	select GFXUMA
 
+if NORTHBRIDGE_VIA_VX900
+
 config MAX_PIRQ_LINKS
 	int
 	default 8
@@ -39,3 +41,5 @@ config MMCONF_BASE_ADDRESS
 config VGA_BIOS_ID
 	string
 	default "1106,7122"
+
+endif