powerpc/microwatt: Add mmu bits to device tree

In commit 5402e239 ("powerpc/64s: Get LPID bit width from device
tree") the kernel tried to determine the pid and lpid bits from the
device tree. If they are not found, there is a fallback, but Microwatt
wasn't covered as it has the unusual configuration of being both !HV and
bare metal.

Set the values in the device tree to avoid having to add a special case.
The lpid value is the only one required, but add both for completeness.

Fixes: 5402e239 ("powerpc/64s: Get LPID bit width from device tree")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220519125706.593532-1-joel@jms.id.au