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>

tests/tcg/riscv64/Makefile.target

@@ -17,4 +17,4 @@ run-test-aes: QEMU_OPTS += -cpu rv64,zk=on
 TESTS += test-fcvtmod
 test-fcvtmod: CFLAGS += -march=rv64imafdc
 test-fcvtmod: LDFLAGS += -static
-run-test-fcvtmod: QEMU_OPTS += -cpu rv64,d=true,Zfa=true
+run-test-fcvtmod: QEMU_OPTS += -cpu rv64,d=true,zfa=true