thermal: int340x: Limit Kconfig to 64-bit

thermal: int340x: Limit Kconfig to 64-bit

32-bit processors cannot generally access 64-bit MMIO registers
atomically, and it is unknown in which order the two halves of
this registers would need to be read:

drivers/thermal/intel/int340x_thermal/processor_thermal_mbox.c: In function 'send_mbox_cmd':
drivers/thermal/intel/int340x_thermal/processor_thermal_mbox.c:79:37: error: implicit declaration of function 'readq'; did you mean 'readl'? [-Werror=implicit-function-declaration]
   79 |                         *cmd_resp = readq((void __iomem *) (proc_priv->mmio_base + MBOX_OFFSET_DATA));
      |                                     ^~~~~
      |                                     readl

The driver already does not build for anything other than x86,
so limit it further to x86-64.

Fixes: aeb58c860dc5 ("thermal/drivers/int340x: processor_thermal: Suppot 64 bit RFIM responses")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>