net: phy: realtek: add dt property to enable ALDPS mode

If enable Advance Link Down Power Saving (ALDPS) mode, it will change
crystal/clock behavior, which cause RXC clock stop for dozens to hundreds
of miliseconds. This is comfirmed by Realtek engineer. For some MACs, it
needs RXC clock to support RX logic, after this patch, PHY can generate
continuous RXC clock during auto-negotiation.

ALDPS default is disabled after hardware reset, it's more reasonable to
add a property to enable this feature, since ALDPS would introduce side effect.
This patch adds dt property "realtek,aldps-enable" to enable ALDPS mode
per users' requirement.

Jisheng Zhang enables this feature, changes the default behavior. Since
mine patch breaks the rule that new implementation should not break
existing design, so Cc'ed let him know to see if it can be accepted.

Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>