riscv: Unify Qemu's reset vector code path

Currently, all riscv machines except sifive_u have identical reset vector
code implementations with memory addresses being different for all machines.
They can be easily combined into a single function in common code.

Move it to common function and let all the machines use the common function.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20200701183949.398134-2-atish.patra@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>