nicole: dts: Add RCLK to MAC clocks

Fixes build with kernel 5.4.4+

This is a copy of the change which was done in the linux repo for
Romulus and other AST2500 based HW

Original commit is 06a318f9  Andrew Jeffery <andrew@aj.id.au>
ARM: dts: aspeed: Add RCLK to MAC clocks for RMII interfaces

We need to ungate RCLK on AST2500- and AST2600-based platforms for RMII
to function. RMII interfaces are commonly used for NCSI.

(From meta-yadro rev: d971ee2180f448f125fb4e9e471cd10a61bf5ff8)

Change-Id: If92e32f144b0c79b937def0136cbf86b4a8aeb73
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>