arm: dts: am335x: add 'u-boot, dm-pre-reloc' to panel

Add the "u-boot,dm-pre-reloc" property to the "ti,tilcdc,panel"
compatible node. In this way the video-uclass module can allocate the
amount of memory needed to be assigned to the frame buffer.
For boards that support Linux the property is added to the *-u-boot.dtsi
file since it is a u-boot specific dt flag.

Ran building tests with CONFIG_AM335X_LCD enabled and disabled for the
following configurations:
 - brxre1_defconfig           --> success
 - am335x_guardian_defconfig  --> success
 - am335x_evm_defconfig       --> success
 - da850evm_defconfig         --> failure with CONFIG_AM335X_LCD enabled

Enabling CONFIG_AM335X_LCD in da850evm_defconfig causes building errors
even without applying the patch. The driver has never been enabled on the
da850 and must be adapted for this platform.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Tested-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Felix Brack <fb@ltec.ch>