rockchip: board: remove SCLK_GPU from U-Boot DT

Starting with commit 92f1e9a4 ("clk: Detect failure to set
defaults") the clk driver for the PX30 would fail to probe for the
Odroid Go Advance. This patch is to remove the clock for the GPU from
the U-Boot specific devicetree, as that clock is not supported by the
U-Boot clk_px30 driver.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>