The current ast2600 I3C clock definitions are top-level (rather than
based on their actual hw sources: either HCLK or APLL), and include a
couple of definitions for (non-existent) i3c6 and i3c7.

Re-parent the individual I3C controller clocks to the main i3c clock,
explicitly sourced from the APLL rather than whatever G6_CLK_SELECTION5
was last set to.

While we're at it, remove the definitions for the i3c6 and i3c7 clock
lines; this hardware isn't present.

This is a partial cherry-pick and rework of ed44b8cdfdb and 1a35eb926d7
from Aspeed's own tree, originally by Dylan Hung
<dylan_hung@aspeedtech.com>.
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20230302005834.13171-4-jk@codeconstruct.com.auReviewed-by: Joel Stanley <joel@jms.id.au>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>