riscv: Do some basic architecture level cpu initialization

riscv: Do some basic architecture level cpu initialization

In arch_cpu_init_dm() do some basic architecture level cpu
initialization, like FPU enable, etc.

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