DaVinci: rename setup_usb() to davinci_setup_usb()

Rename setup_usb() into davinci_setup_usb().  While at it:

- move its declaration from <mach/common.h> to more fitting <mach/usb.h>;
- teach it to handle values of the 'mA' parameter greater than 510 and thus
  pass 1000 instead of 500 for the power switches capable of sourcing over 1 A;
- teach it to handle odd values of the 'potpgt_ms' parameter...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>