MAINTAINERS: Add memory drivers to AT91 entry

Both atmel-ebi and atmel-sdram are maintained as part of the Atmel ARM
SoCs.

Suggested-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>