MAINTAINERS: add new entries for Armada 37xx cpufreq driver

This new driver belongs to the mvebu family, update the MAINTAINER file
to document it.
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

MAINTAINERS

@@ -1582,6 +1582,7 @@ F:	arch/arm/boot/dts/kirkwood*
 F:	arch/arm/configs/mvebu_*_defconfig
 F:	arch/arm/mach-mvebu/
 F:	arch/arm64/boot/dts/marvell/armada*
+F:	drivers/cpufreq/armada-37xx-cpufreq.c
 F:	drivers/cpufreq/mvebu-cpufreq.c
 F:	drivers/irqchip/irq-armada-370-xp.c
 F:	drivers/irqchip/irq-mvebu-*