MAINTAINERS: Move Vivien to CREDITS

Last patch from Vivien was nearly 3 years ago and he has not reviewed or
responded to DSA patches since then, move to CREDITS.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20221109231907.621678-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>