ramips: DTS modifications

ramips: DTS modifications

This commit makes the following modifications to ramips dts files:
1. Add clkctrl node to all dtsi files (although not used for now)
2. Add clocks and clock-names properties to some nodes (usbphy, pci)
3. Add usbphy node for rt3050 (although not used for now)
4. Add clock-frequency to uart nodes in mt7621.dtsi and mt7628an.dtsi

These modifications, although not fully used at the moment, will make
it easier for FreeBSD to adopt and use LEDE ramips dts files with
minimal changes for easier maintenance.

Signed-off-by: Stanislav Galabov <sgalabov@gmail.com>