ARM: i.MX: Move SOC_IMX1 into 'Device tree only'

This patch moves SOC_IMX1 into 'Device tree only' category and renames
imx1-dt.c to mach-imx1.c to align with the name schema of other
i.MX DT only platforms.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>