hw/timer/armv7m_systick: Add usual QEMU interface comment

Add the usual-style QEMU interface comment documenting what
properties, etc, this device exposes.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-5-peter.maydell@linaro.org

include/hw/timer/armv7m_systick.h

@@ -20,6 +20,13 @@
 
 OBJECT_DECLARE_SIMPLE_TYPE(SysTickState, SYSTICK)
 
+/*
+ * QEMU interface:
+ *  + sysbus MMIO region 0 is the register interface (covering
+ *    the registers which are mapped at address 0xE000E010)
+ *  + sysbus IRQ 0 is the interrupt line to the NVIC
+ */
+
 struct SysTickState {
     /*< private >*/
     SysBusDevice parent_obj;