arc: axs10x - add support of ARC PGU

Synopsys DesignWare ARC SDP boards sport ARC SDP display
controller attached to ADV7511 HDMI encoder.

That change adds desctiption of both ARC PGU and ADV7511 in
ARC SDP'd base-board Device Tree.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: devicetree@vger.kernel.org
Cc: linux-snps-arc@lists.infradead.org