pl031: Expose RTCICR as proper WC register

The current PL031 RTCICR register implementation always clears the
IRQ pending status on a register write, regardless of the value the
guest writes.

To justify that behavior, it references the ARM926EJ-S Development
Chip Reference Manual (DDI0287B) and indicates that said document
states that any write clears the internal IRQ state.  It is indeed
true that in section 11.1 this document says:

  "The interrupt is cleared by writing any data value to the
   interrupt clear register RTCICR".

However, later in section 11.2.2 it contradicts itself by saying:

  "Writing 1 to bit 0 of RTCICR clears the RTCINTR flag."

The latter statement matches the PL031 TRM (DDI0224C), which says:

  "Writing 1 to bit position 0 clears the corresponding interrupt.
   Writing 0 has no effect."

Let's assume that the self-contradictory DDI0287B is in error, and
follow the reference manual for the device itself, by making the
register write-one-to-clear.

Reported-by: Hendrik Borghorst <hborghor@amazon.de>
Signed-off-by: Alexander Graf <graf@amazon.com>
Message-id: 20191104115228.30745-1-graf@amazon.com
[PMM: updated commit message to note that DDI0287B says two
 conflicting things]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/rtc/pl031.c

@@ -149,11 +149,7 @@ static void pl031_write(void * opaque, hwaddr offset,
         pl031_update(s);
         break;
     case RTC_ICR:
-        /* The PL031 documentation (DDI0224B) states that the interrupt is
-           cleared when bit 0 of the written value is set.  However the
-           arm926e documentation (DDI0287B) states that the interrupt is
-           cleared when any value is written.  */
-        s->is = 0;
+        s->is &= ~value;
         pl031_update(s);
         break;
     case RTC_CR: