ARM: dts: imxrt1020-evk: move all u-boot, dm-spl to imxrt1020-evk-u-boot.dtsi file

ARM: dts: imxrt1020-evk: move all u-boot, dm-spl to imxrt1020-evk-u-boot.dtsi file

At the moment a lot of u-boot,dm-spl properties are present in board .dts
file but this is not correct since u-boot,dm-spl property is u-boot
specific and must be listed into the separate imrt1020-evk-u-boot.dtsi
file. So let's move every u-boot,dm-spl property present in
imxrt1020-evk.dts to imxrt1020-evk-u-boot.dtsi file.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>