ARM: add the imx31_phycore_eet target to MAINTAINERS

imx31_phycore_eet is a variant of the imx31_phycore board with a few
extensions, which justifies a separate entry in the MAINTAINERS list,
whereas normally all entries sharing a single configuration file and a
board/ directory have only one entry in MAINTAINERS.

Reported-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Guennadi Liakhovetski <lg@denx.de>