Enable Apex on ARM64, fix compile issue

- Change from X86 only to X86 or ARM64 in Kconfig
- refcount_read doesn't exist in 4.11, so use atomic_read instead. This
is the same call that refcount_read in 4.12+ would make.

Change-Id: I48c97dd8c14136dcccaa8378b3a931a7872e1289

Kconfig

@@ -2,7 +2,7 @@ menu "Gasket devices"
 
 config STAGING_GASKET_FRAMEWORK
 	tristate "Gasket framework"
-	depends on PCI && X86_64
+	depends on PCI && (X86_64 || ARM64)
 	help
 	  This framework supports Gasket-compatible devices, such as Apex.
 	  It is required for any of the following module(s).

gasket_sysfs.c

@@ -108,7 +108,7 @@ static void put_mapping(struct gasket_sysfs_mapping *mapping)
 	}
 
 	mutex_lock(&mapping->mutex);
-	if (refcount_read(&mapping->refcount.refcount) == 0)
+	if (atomic_read(&mapping->refcount.refcount) == 0)
 		gasket_nodev_error("Refcount is already 0!");
 	if (kref_put(&mapping->refcount, release_entry)) {
 		gasket_nodev_info("Removing Gasket sysfs mapping, device %s",