[kernel] Pass the fb phys addr to userspace

Instead of always mapping the framebuffer at an arbitrary location, and
so reporting that to userspace, send the physical address so drivers can
call system_map_mmio().

src/include/j6/init.h

@@ -28,7 +28,7 @@ struct j6_init_value {
 /// `flags` has the following bits:
 ///   0-3:   Pixel layout. 0000: rgb8, 0001: bgr8
 struct j6_init_framebuffer {
-	void* addr;
+	uintptr_t addr;
 	size_t size;
 	uint32_t vertical;
 	uint32_t horizontal;