MAINTAINERS: Add Pratyush Anand as SPEAr13xx and DesignWare PCIe maintainer

Didn't add myself as Maintainer earlier, as I was moving out of ST
and didn't had a board to test. But learnt recently that, it's fine being
in MAINTAINERS in such situations, at least I can review the patches.

So, let me add myself.

Signed-off-by: Pratyush Anand <pratyush.anand@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>