mips: bmips: bcm6368: populate device tree nodes

- Rename periph_clk to periph_osc.
- Rename clkctl to periph_clk.
- Move syscon-reboot to subnode.
- Add watchdog controller.
- Add SPI controller.
- Add NAND controller.
- Add USBH PHY controller.
- Add RNG controller.
- Add cfi-flash controller.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>