dts: rockchip: rk3308: Avoid warning for serial probe on prereloc

Make device tree complete and consistent for pre relocation phase. Some
nodes are missing, causing warnings to be issued on serial port probing
during pre relocation phase (uclass_get_device_by_phandle_id fails when
called by pinctrl_select_state_full: none of these failures is fatal
nor causing issues). Add to *-u-boot.dtsi all required nodes with the
'bootph-some-ram' attribute.

Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>