tests: riscv64: Use 'zfa' instead of 'Zfa'

tests: riscv64: Use 'zfa' instead of 'Zfa'

Running test-fcvtmod triggers the following deprecation warning:
  warning: CPU property 'Zfa' is deprecated. Please use 'zfa' instead
Let's fix that.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240229180656.1208881-1-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>