rockchip: rk3399: spl: remove unused header inclusion

fdtdec.h is included, but not used in rk3399-board-spl.c: remove the
'#include'-statement.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/mach-rockchip/rk3399-board-spl.c

@@ -7,7 +7,6 @@
 #include <common.h>
 #include <debug_uart.h>
 #include <dm.h>
-#include <fdtdec.h>
 #include <ram.h>
 #include <spl.h>
 #include <asm/gpio.h>