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>