KVM: PPC: Book3S HV nestedv2: Add DPDES support in helper library for Guest state buffer

Add support for using DPDES in the library for using guest state
buffers. DPDES support is needed for enabling usage of doorbells in a L2
KVM on PAPR guest.

Fixes: 6ccbbc33 ("KVM: PPC: Add helper library for Guest State Buffers")
Cc: stable@vger.kernel.org # v6.7+
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240605113913.83715-2-gautam@linux.ibm.com