m32x: make PWM timer interrupt interval bits read-only from the MD side (#1813)

Per page 24 of the 32X hardware manual, the PWM timer interrupt interval
is read-only from the Mega Drive side.

This change fixes incorrect sound effects in Primal Rage. If the 68000
is allowed to write to these bits, it will change the PWM timer interval
from 1 to 16 during game boot, which breaks the game's PWM sound driver
code.

Primal Rage title screen audio before this change:

[primal-rage-before.webm](https://github.com/user-attachments/assets/0d8a33d1-22fb-4664-a802-00715541e67f)

After:

[primal-rage-after.webm](https://github.com/user-attachments/assets/87a3cdfe-85a4-45e7-8ea8-bc3e5bde4f7a)

Author: jsgroth

ares/md/m32x/io-external.cpp

@@ -267,8 +267,8 @@ auto M32X::writeExternalIO(n1 upper, n1 lower, n24 address, n16 data) -> void {
     //pwm.dreqIRQ = data.bit(7) = readonly;
     }
     if(upper) {
-      pwm.timer = data.bit(8,11);
-      pwm.periods = 0;
+    //pwm.timer = data.bit(8,11) = readonly;
+    //pwm.periods = 0 = readonly;
     }
   }