riscv: make use of the barrier functions from Linux

Replace the barrier functions in arch/riscv/include/asm/io.h with those
defined in barrier.h, which is imported from Linux. This version is
modified to remove the include statement of asm-generic/barrier.h, which
is not available in U-Boot or required.

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