MAINTAINERS: reinstate freescale ARM64 DT directory in i.MX entry

The MAINTAINERS entry's F: currently only matches the 32-bit device trees,
as commit 724ba675 ("ARM: dts: Move .dts files to vendor sub-directories")
inadvertently dropped the 64-bit DT match when it added the 32 bit
matches. The entry has a N: imx, which reduced the impact a bit, but
still some board device trees may not contain the substring and would
thus not be covered by the entry.

Reinstate the missing F: line to restore previous behavior.

Fixes: 724ba675 ("ARM: dts: Move .dts files to vendor sub-directories")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>