dt-bindings: Remove VSC8531 specific RGMII delay definitions

Based on Linux upstream discussion value enumeration shouldn't be used.
Instead of it delay in pS should be used that's why remove it from the
header.

Link: https://lore.kernel.org/all/YNsm%2F0dmpBgO8mqr@lunn.ch/


Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/bbd343474856a67252f3b31d22b3b5a80ad04043.1698851109.git.michal.simek@amd.com