MAINTAINERS: Add SoC drivers to AT91 entry

We (the AT91/Atmel SoC maintainers) are not seeing patches for several
drivers present only on our SoCs. Add more patterns to match at91 and atmel
drivers.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>