MAINTAINERS: Add Siddharth Vadapalli as PCI TI DRA7XX/J721E reviewer

Since I have been contributing to the driver for a while and wish to help
with the review process, add myself as a reviewer.

Link: https://lore.kernel.org/r/20240216065926.473805-1-s-vadapalli@ti.com
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>