powerpc/5200: update contact email

My 'pengutronix' address is defunct for years. Merge the entries and use
the proper contact address.

Signed-off-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200502142642.18979-1-wsa@kernel.org