usb: stm32: added support for USB Device mode on STM32F105xx parts

The STM32F105xx USB clock goes through a prescaler either PLL1 x2 /2
or PLL1 x2 /3, the output of this must be 48 MHz. As such, the output
of PLL1 must be 48 MHz or 72 MHz.

NOTE: This requires that the system is running from PLL1 (PLLCLK).
      Support for running from PLL2 will be implemented in the future.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>