target/arm: Use uint64_t for midr field in CPU state struct

MIDR_EL1 is a 64-bit system register with the top 32-bit being RES0.
Represent it in QEMU's ARMCPU struct with a uint64_t, not a
uint32_t.

This fixes an error when compiling with -Werror=conversion
because we were manipulating the register value using a
local uint64_t variable:

  target/arm/cpu64.c: In function ‘aarch64_max_initfn’:
  target/arm/cpu64.c:628:21: error: conversion from ‘uint64_t’ {aka ‘long unsigned int’} to ‘uint32_t’ {aka ‘unsigned int’} may change value [-Werror=conversion]
    628 |         cpu->midr = t;
        |                     ^

and future-proofs us against a possible future architecture
change using some of the top 32 bits.

Suggested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20200428172634.29707-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu.c

@@ -2755,7 +2755,7 @@ static const ARMCPUInfo arm_cpus[] = {
 static Property arm_cpu_properties[] = {
     DEFINE_PROP_BOOL("start-powered-off", ARMCPU, start_powered_off, false),
     DEFINE_PROP_UINT32("psci-conduit", ARMCPU, psci_conduit, 0),
-    DEFINE_PROP_UINT32("midr", ARMCPU, midr, 0),
+    DEFINE_PROP_UINT64("midr", ARMCPU, midr, 0),
     DEFINE_PROP_UINT64("mp-affinity", ARMCPU,
                         mp_affinity, ARM64_AFFINITY_INVALID),
     DEFINE_PROP_INT32("node-id", ARMCPU, node_id, CPU_UNSET_NUMA_NODE_ID),

target/arm/cpu.h

@@ -894,7 +894,7 @@ struct ARMCPU {
         uint64_t id_aa64dfr0;
         uint64_t id_aa64dfr1;
     } isar;
-    uint32_t midr;
+    uint64_t midr;
     uint32_t revidr;
     uint32_t reset_fpsid;
     uint32_t ctr;