MAINTAINERS: Add Vignesh to TI K3 platform maintainership

Add Vignesh to TI K3 platform maintainership. We will rotate roles for
each kernel version to help spread the work load a little better.

Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210915121308.26795-1-nm@ti.com'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>