docker: install riscv32 toolchain

docker: install riscv32 toolchain

For building riscv32 targets we should use the riscv32 toolchain.
Add it to the Docker image.

Drop the riscv toolchain-alias as we do not need it in future.

While in here, update to the latest "jammy" tag.

Signed-off-by: Heinrich Schuchardt <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Rick Chen <[email protected]>
[trini: Update to latest jammy tag]
Signed-off-by: Tom Rini <[email protected]>