realtek: dsa: rtl93xx: allow to adjust LED_CLK

realtek: dsa: rtl93xx: allow to adjust LED_CLK

The LED_GLB_CTRL register has a field to select the clock used for LED
output, either for serial LED output or for the external auxiliary MDIO
to connect a RTL8231. While the default value for the period time of
400ns is the correct one for most applications, some special devices
require a dedicated setting, e.g. UniFi switches with Etherlighting.

Support the generic DT property "clock-frequency" within the LED node,
and read + apply during LED configuration. In case the property isn't
specified, no change is applied to keep previous behavior. In case an
unsupported value is supplied, the default is used as a fallback

Link: https://github.com/openwrt/openwrt/pull/23944
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>