Documentation: embargoed-hardware-issues.rst: Add myself for Power

Unfortunately Anton has left IBM. Add myself as the contact for Power,
until someone else volunteers.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20240322103840.668746-1-mpe@ellerman.id.au
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>