mfd: omap-usb-tll: Provide description for 'pdev' argument to .probe()

Kerneldoc syntax is used, but not complete.  Arg descriptions required.

Prevents warnings like:

 drivers/mfd/omap-usb-tll.c:204: warning: Function parameter or member 'pdev' not described in 'usbtll_omap_probe

Cc: Tony Lindgren <tony@atomide.com>
Cc: Keshava Munegowda <keshava_mgowda@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>