riscv: Move the linker script to the CPU root directory

The linker script can be shared by all RISC-V targets. Move it to
a common place.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>

diff --git a/arch/riscv/cpu/ax25/u-boot.lds b/arch/riscv/cpu/u-boot.lds
similarity index 100%
rename from arch/riscv/cpu/ax25/u-boot.lds
rename to arch/riscv/cpu/u-boot.lds