dt-bindings: amlogic: Add support for GXL S805X and the P241 board

The S805X is a variant of the Amlogic Meson GXL SoC family with the
following physical limitations :
- No SDCard interface
- No GPIOCLK pins
- No USB OTG ID pin (but Gadget feature can stil be forced)
- No 5V regulator
- Reduced ADC inputs (only ADC0 and ADC1)
- GPIODV_26, GPIOAO_3, GPIOAO_4, GPIOAO_5, GPIOAO_6, GPIOX_14,
  GPIOX_15, GPIOH_3 are no more exposed on the package

Amlogic exposes the following SW limitations :
- HDMI max resolution should be 1080p60, VPU clock should be downgraded
- Video Decoding should be limited to 1080p60, VDEC clock should be downgraded
- CPU speed should be limited by SCPI OPP table to 1.2GHz
- DRAM interface is limited to DDR4 16bit up to 1GiB

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>