MAINTAINERS: Move Jason Cooper to CREDITS

Jason's email address has now been bouncing for weeks, and no
reply was received when trying to reach out on other addresses.

We really hope he is OK. But until we hear of his whereabouts,
let's move him to the CREDITS file so that people stop Cc-ing
him.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201128103707.332874-1-maz@kernel.org