board: phytec: phycore-imx93: Add phyBOARD-Segin-i.MX93 support

Add initial support for the PHYTEC phyBOARD-Segin-i.MX93 board based on
the PHYTEC phyCORE-i.MX93 SoM.

Supported features:
- 1GB LPDDR4 RAM
- eMMC
- external SD
- FEC Ethernet
- debug UART
- watchdog

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>
Tested-by: Primoz Fiser <primoz.fiser@norik.com>
Reviewed-by: Yannic Moog <y.moog@phytec.de>