usb: gadget: composite: enable BESL support

According to USB 2.0 ECN Errata for Link Power
Management (USB2-LPM-Errata-final.pdf), BESL
must be enabled if LPM is enabled.

This helps with USB30CV TD 9.21 LPM L1
Suspend Resume Test.

Signed-off-by: Felipe Balbi <balbi@ti.com>