hw/arm/sbsa-ref: set 'slots' property of xhci

hw/arm/sbsa-ref: set 'slots' property of xhci

This extends the slots of xhci to 64, since the default xhci_sysbus
just supports one slot.

Signed-off-by: Wang Yuquan <wangyuquan1236@phytium.com.cn>
Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-id: 20230710063750.473510-2-wangyuquan1236@phytium.com.cn
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>