sys/dts: Remove i.MX5 device tree

IMX51 is already EOL and IMX53 is going to be EOL in Feb 2026.
See NXP product longevity site

Remove
Digi ConnectCore Wi-i.MX53
Genesi Efika MX
FreeBSD own DTSI for imx51 and imx53
sys/modules/dtb/imx5 for i.MX53 quick start board.

Approved by: manu (mentor)
Differential revision: https://reviews.freebsd.org/D53888