configs/beaglev: switch to upstream opensbi generic platform

configs/beaglev: switch to upstream opensbi generic platform

Switch from the starfive-tech opensbi to upstream repo and use generic
platform now that it supports the console uart [1]. This landed after
the 0.9 release [2], so the current head of master branch is used [3].

FW_FDT_PATH is set to u-boot.dtb so that the device tree from U-Boot
will be included in the OpenSBI build [4]. Note that U-Boot must have
CONFIG_OF_SEPARATE enabled for U-Boot to build u-boot.dtb [5].

[1] https://github.com/riscv/opensbi/commit/e822b7504df0583a6a227d51cb7da8ea9fc79c0a
[2] https://github.com/riscv/opensbi/commit/234ed8e427f4d92903123199f6590d144e0d9351
[3] https://github.com/riscv/opensbi/commit/79f9b4220ffa7f74356054be25d450d7958bf16c
[4] https://github.com/riscv/opensbi/blob/master/docs/firmware/fw.md
[5] https://github.com/u-boot/u-boot/blob/master/doc/README.fdt-control

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>