MAINTAINERS: Add vaishnavachath as maintainer for TI Platforms

Add myself as a maintainer for existing TI platforms,
currently I am an employee with Texas Instruments Inc. in
the Embedded Processors software department and has worked
on the supported simplelink platforms in Zephyr for the last
2 years.

Signed-off-by: Vaishnav Achath <vaishnav.a@ti.com>