MAINTAINERS: alpine: add a new maintainer and update the entry

Add myself as a co-maintainer for the Alpine support. Also update the
entry to take in account Alpine ARM64 boards, Alpine ARM device trees
and Alpine-specific drivers.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Tsahee Zidenberg <tsahee@annapurnalabs.com>
Signed-off-by: Olof Johansson <olof@lixom.net>