MAINTAINERS: Add patterns for DTS files for AT91

Help ensure updates to the DTS files for AT91 machines and devices get
sent to the maintainers by adding patterns to the MAINTAINERS entry for
the architecture.

Signed-off-by: Mark Brown <broonie@linaro.org>
[nicolas.ferre@atmel.com: add "sama*.dts/dtsi" strings to the list]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>