riscv: qemu: Implement is_flash_available() for MTD NOR

Currently, if MTD NOR is enabled then U-Boot tries to issue flash
commands even when CFI flash DT node is not present. This causes
access fault on RISC-V emulators or ISS which do not emulate CFI
flash. To handle this issue, we implement is_flash_available() for
qemu-riscv board which will return 1 only if CFI flash DT node is
present.

Fixes: d248627f ("riscv: qemu: Enable MTD NOR flash support")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>