xen/xenpv: remove low memory limit for non-x86

For embedded devices reserved addresses will be known in advance.  More
recently added devices will also likely be correctly updated.  As a
result using any available address is reasonable on non-x86.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29304

sys/dev/xen/bus/xenpv.c

@@ -55,13 +55,17 @@ __FBSDID("$FreeBSD$");
  *
  * Since this is not possible on i386 just use any available memory
  * chunk above 1MB and hope we don't clash with anything else.
+ *
+ * Other architectures better document MMIO regions and drivers more
+ * reliably reserve them.  As such, allow using any unpopulated memory
+ * region.
  */
 #ifdef __amd64__
 #define LOW_MEM_LIMIT	0x100000000ul
 #elif defined(__i386__)
 #define LOW_MEM_LIMIT	0x100000ul
 #else
-#error "Unsupported architecture"
+#define LOW_MEM_LIMIT	0
 #endif
 
 static devclass_t xenpv_devclass;