usb: Move SPL_USB_HOST Kconfig symbol 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_HOST and updates help text of both USB_HOST
and SPL_USB_HOST .

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