MAINTAINERS: Add Kishon as maintainer of PHY bindings

DT bindings normally go in via subsystem maintainers, so add PHY
bindings under generic PHY framework.

Reported-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>