usb: dwc3: make LPM configurable in DT

This patch removes "Enable USB3 LPM Capability" option from Kconfig
and adds snps,usb3_lpm_capable devicetree property instead of it.

USB3 LPM (Link Power Management) capability is hardware property, and
it's platform dependent, so if our hardware supports this feature, we
want rather to configure it in devicetree than having it as Kconfig option.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>