ACPI: fix ACPI=n allmodconfig build

Today's -tip failed to build because commit
9e368fa0

 ("ipmi: add PNP discovery (ACPI
namespace via PNPACPI)") from today's upstream kernel causes the following
build failure on x86, for CONFIG_ACPI=n && CONFIG_IPMI_SI=y:

 drivers/char/ipmi/ipmi_si_intf.c:3208: error: 'ipmi_pnp_driver' undeclared (first use in this function)
 drivers/char/ipmi/ipmi_si_intf.c:3208: error: (Each undeclared identifier is reported only once
 drivers/char/ipmi/ipmi_si_intf.c:3208: error: for each function it appears in.)
 drivers/char/ipmi/ipmi_si_intf.c:3334: error: 'ipmi_pnp_driver' undeclared (first use in this function)

The reason is that the ipmi_pnp_driver depends on ACPI facilities and is only
made available under ACPI - while the registration and unregistration is made
dependent on CONFIG_PNP:

 #ifdef CONFIG_PNP
 	pnp_register_driver(&ipmi_pnp_driver);
 #endif

The solution is to only register this driver under ACPI. (Also, the CONFIG_PNP
dependency is not needed because pnp_register_driver() is stubbed out in the
!CONFIG_PNP case.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Myron Stowe <myron.stowe@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>