usb: Move SPL_USB_GADGET and related Kconfig symbols to drivers/usb/

To avoid piling up all the various Kconfig symbols in one place, i.e.
common/spl/Kconfig, move the USB Kconfig symbols into drivers/usb/ .
This commit moves SPL_USB_GADGET and related symbols. Fix typo and
rename SPL_USB_GADGET to "USB Gadget Support in SPL" .

Update the gadget Makefile to match the symbol changes.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>