Fix pointer-to-int-cast compiler warning

A void pointer(fast_memory) was type casted to uint64_t. This resulted
in a compiler warning [-Wpointer-to-int-cast]. This patch fixes the
warning.

Change-Id: I61d4a77664c7a2af7b9020f8a98c10d27c78334f
Signed-off-by: Rajasekaran Kalidoss <rajasekaran.kalidoss@arm.com>
diff --git a/src/ethosu_driver.c b/src/ethosu_driver.c
index dead0b9..5128455 100644
--- a/src/ethosu_driver.c
+++ b/src/ethosu_driver.c
@@ -425,7 +425,7 @@
         }
     }
 
-    drv->fast_memory           = (uint64_t)fast_memory;
+    drv->fast_memory           = (uintptr_t)fast_memory;
     drv->fast_memory_size      = fast_memory_size;
     drv->power_request_counter = 0;