ARC: axs10x: really enable ARC PGU

Up until now we had ARC PGU not enabled in axs10x defconfigs trying
to not bloat kernel image again with yet another drivers and subsystems.

This change configures ARC PGU (as well as DRM bits it depends on)
to be built as a module and so those who need LCD screen to work on
axs10x may bundle built .ko files in their target's file-system with
help of the following command on host:
------------->8-------------
make INSTALL_MOD_PATH=_path_to_target_fs_ modules_install
------------->8-------------

and later on target with commands as simple as:
------------->8-------------
modprobe adv7511.ko
modprobe arcpgu.ko
------------->8-------------
get LCD working.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>