usb: musb: dsps: document dt bindings properly

DT bindings normally use '-' (hyphens) instead of '_' (underscore),
driver has it the proper way, but binding documentation does not
reflect it, fix it.
Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/am33xx-usb.txt

@@ -3,12 +3,12 @@ AM33XX MUSB GLUE
  - ti,hwmods : must be "usb_otg_hs"
  - multipoint : Should be "1" indicating the musb controller supports
    multipoint. This is a MUSB configuration-specific setting.
- - num_eps : Specifies the number of endpoints. This is also a
+ - num-eps : Specifies the number of endpoints. This is also a
    MUSB configuration-specific setting. Should be set to "16"
- - ram_bits : Specifies the ram address size. Should be set to "12"
- - port0_mode : Should be "3" to represent OTG. "1" signifies HOST and "2"
+ - ram-bits : Specifies the ram address size. Should be set to "12"
+ - port0-mode : Should be "3" to represent OTG. "1" signifies HOST and "2"
    represents PERIPHERAL.
- - port1_mode : Should be "1" to represent HOST. "3" signifies OTG and "2"
+ - port1-mode : Should be "1" to represent HOST. "3" signifies OTG and "2"
    represents PERIPHERAL.
  - power : Should be "250". This signifies the controller can supply upto
    500mA when operating in host mode.