powerpc/powernv: Simplify definitions of EEH debugfs handlers

The EEH debugfs handlers have same prototype. This introduces
a macro to define them, then to simplify the code. No logical
changes.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>