Revert the read register from 64 bits into 32 bits

Renode gdb server calls function GetRegisterUnsafe to get register data
when we run command `info registers` from the gdb client. The return
data type is UInt64 format for each register. And riscv gdb client will
receive double sized packet and report size error.

Reverting the register size enables us to use riscv gdb running commands
`info threads`, `thread 2` and `info registers` to dump register info on
CHERIot SMC core.

Bug: 331940020
Bypass-Presubmit-Reason: It is a config change to Sencha project.
Matcha is unaffected.

Change-Id: I372b21aca0afb9e3dd0662d8beb5632f40e01eef
diff --git a/shodan_infrastructure/MpactCheriotCPU.cs b/shodan_infrastructure/MpactCheriotCPU.cs
index ba3ca12..b115bad 100644
--- a/shodan_infrastructure/MpactCheriotCPU.cs
+++ b/shodan_infrastructure/MpactCheriotCPU.cs
@@ -328,7 +328,7 @@
             LogAndThrowRE("Failed to read register " + register);
         }
         Int64 value = Marshal.ReadInt64(value_ptr);
-        return (UInt64)value;
+        return (UInt32)value;
     }
 
     private void GetMpactRegisters() {