ARM: dts: at91: add CalAmp LMU5000 board

Add CalAmp LMU5000 board. The base device tree is from OpenWrt tree and
with the addition of this patch there will be no need to maintain it
separatelly in OpenWrt.

[porter.adam: original author of patch in OpenWrt]
[claudiu.beznea: fixed compilation warnings, use &<lable> syntax,
 sorted nodes in alphabetical order, adapted flash to new support
 in kernel 5.14, use proper compatibles according to kernel 5.14,
 use macros instead of hard coded numbers for pinctrl phandles and
 for all pinctrl references, add pinctrl-names, pinctrl-X where
 necessary]

Signed-off-by: Adam Porter <porter.adam@gmail.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210816064416.1630674-5-claudiu.beznea@microchip.com