rockchip: Move stdint header to the offending header file

The stdint header was introduced to rk3399's plat_sip_calls.c in order
to fix missing stdint definitions. However, ordering headers
alphabetically caused the fix to be ineffective, as stint was then
included after the offending header file (dfs.h).

Move the stdint include to that header to properly fix the issue.

Change-Id: Ieaad37a7932786971488ab58fc5b169bfa79e197
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>