MAINTAINERS: update Amitkumar's email address

His email is bouncing, and he'd like to use this new one.

Cc: Amitkumar Karwar <amitkarwar@gmail.com>
Cc: Nishant Sarmukadam <nishants@marvell.com>
Cc: Ganapathi Bhat <gbhat@marvell.com>
Cc: Xinming Hu <huxm@marvell.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>