spl: Fix compilation warnings for arm64

Make code 64bit aware.

Warnings:
+../arch/arm/lib/spl.c: In function ‘jump_to_image_linux’:
+../arch/arm/lib/spl.c:63:3: warning: cast to pointer from integer of
different size [-Wint-to-pointer-cast]
+../common/spl/spl_fat.c: In function ‘spl_load_image_fat’:
+../common/spl/spl_fat.c:91:33: warning: cast to pointer from integer
of different size [-Wint-to-pointer-cast]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>