dt-bindings: net: cdns,macb: replace cdns,refclk-ext with cdns,refclk-source

dt-bindings: net: cdns,macb: replace cdns,refclk-ext with cdns,refclk-source

Ryan added cdns,refclk-ext with the intent of decoupling the source of
the reference clock on sama7g5 (and related platforms) from the
compatible. Unfortunately, the default for sama7g5-emac is an external
reference clock, so this property had no effect there, so that
compatibility with older devicetrees is preserved.
Replace cdns,refclk-ext with one that supports both default states and
therefore is usable for sama7g5-emac.

For now, limit it to only the platforms that have USRIO controlled
reference clock selection, but this could be generalised in the future.

The existing property only works on devices that are compatible with
sama7g5-gem, so mark it deprecated, and limit its use to that specific
scenario.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260325-savior-untainted-03057ee0a917@spud
Signed-off-by: Jakub Kicinski <kuba@kernel.org>