drm/aspeed: Use dt matching for default register values

There are minor differences in the values for the threshold value and
the scan line size between families of ASPEED SoC. Additionally the SCU
registers for the output control and scratch registers differ between
families.

This adds device tree matching to parameterise these values, allowing us
to add support for the AST2400 now, and in the future the AST2600.

Reviewed-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209123734.130483-3-joel@jms.id.au