stm32: add new stm32 target

New stm32 target introduces support for stm32mp1 based devices.
For now it includes an initial support of the STM32MP135F-DK device.
The specifications bellow only list supported features.

Specifications
--------------

SOC: STM32MP135FAF7
RAM: 512 MiB
Storage: SD Card
Ethernet: 2x 100 Mbps
Wireless: 2.4GHz Cypress CYW43455 (802.11b/g/n)
LEDs: Heartbeat (Blue)
Buttons: 1x Reset, 1x User (USER2)
USB: 4x 2.0 Type-A

Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://github.com/openwrt/openwrt/pull/16716
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/stm32/base-files/etc/board.d/02_network

@@ -0,0 +1,18 @@
+
+. /lib/functions/uci-defaults.sh
+. /lib/functions.sh
+. /lib/functions/system.sh
+
+board_config_update
+
+board=$(board_name)
+
+case "$board" in
+st,stm32mp135f-dk)
+	ucidef_set_interfaces_lan_wan "eth0" "eth1"
+	;;
+esac
+
+board_config_flush
+
+exit 0