imx7d-pico: Fix the name of the u-boot.dtsi file

Since commit 2f96d4dd ("imx7s/d: synchronise device trees with linux")
the imx7d-pico board no longer boots.

The reason is that prior to the above commit there was an explicit
inclusion of arch/arm/dts/imx7d-pico-u-boot.dtsi inside imx7d-pico.dtsi.

After the syncing with the Linux upstream dtsi, this u-boot.dtsi inclusion
is gone and the board fails to boot.

U-Boot uses the imx7d-pico-pi.dtb file, so rename the u-boot.dtsi to
imx7d-pico-pi-u-boot.dtsi which gets included automatically by U-Boot
standard make logic and makes the board boot again.

Signed-off-by: Fabio Estevam <festevam@denx.de>