Rock-5B-Plus: Use upstream device-tree name

Related to armbian/linux-rockchip@d080e34

Author: HeyMeco

config/boards/rock-5b-plus.conf

@@ -7,7 +7,6 @@ KERNEL_TARGET="edge,vendor"
 KERNEL_TEST_TARGET="vendor,edge"
 FULL_DESKTOP="yes"
 BOOT_LOGO="desktop"
-BOOT_FDT_FILE="rockchip/rk3588-radxa-rock-5b+.dtb"
 BOOT_SCENARIO="spl-blobs"
 BOOT_SUPPORT_SPI="yes"
 BOOT_SPI_RKSPI_LOADER="yes"