[C25][HDMI] enable HDMI video output in U-Boot for modules with enabled HDMI feature

Following logic added to U-Boot:

• enabled HDMI in U-Boot config;
• in device tree, added an HDMI node; routed DCSS to HDMI; defined a power_regulator/switch for HDMI;
• in board file, added a check whether the HDMI feature is present on particular C25 device,
• in board file, added a routine to activate HDMI via corresponding GPIO/power_regulator if the HDMI feature is present.