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

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 <[email protected]>
Suggested-by: Peter Maydell <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Laurent Desnogues <[email protected]>
Message-id: 20200428172634[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>