powerpc/reg: Add definition for LPCR_PECE_HVEE

ISA 3.0 defines a new PECE (Power-saving mode Exit Cause Enable) field
in the LPCR (Logical Partitioning Control Register), called
LPCR_PECE_HVEE (Hypervisor Virtualization Exit Enable).

KVM code will need to know about this bit, so add a definition for it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>