board: phytec: phycore_am62x: Use custom k3_dfu.env fragment

TI's k3_dfu.env includes redundant dfu_alt_info_* data, some of which
is incompatible with our board configuration. Replace it with a custom
variant that better aligns with our setup, ensuring correct offsets and
eliminating unnecessary entries.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>

board/phytec/phycore_am62x/phycore_am62x.env

@@ -1,4 +1,4 @@
-#include <env/ti/k3_dfu.env>
+#include <env/phytec/k3_dfu.env>
 #include <env/phytec/k3_mmc.env>
 #include <env/phytec/k3_net.env>
 #include <env/phytec/k3_spi.env>

include/env/phytec/k3_dfu.env

@@ -0,0 +1,3 @@
+dfu_alt_info_ram=
+	tispl.bin ram 0x80080000 0x200000;
+	u-boot.img ram 0x81000000 0x400000