powerpc/pseries: Update MAX_HCALL_OPCODE to reflect page coalescing

When page coalescing support was added recently, the MAX_HCALL_OPCODE
define was not updated for the newly added H_GET_MPP_X hcall.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>