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>
diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
index 6213b3c..1a883ba 100644
--- a/drivers/usb/host/Kconfig
+++ b/drivers/usb/host/Kconfig
@@ -6,6 +6,19 @@
 config USB_HOST
 	bool
 	select DM_USB
+	help
+	  Enable access to USB (Universal Serial Bus) host devices so that
+	  SPL can load U-Boot from a connected USB peripheral, such as a USB
+	  flash stick. While USB takes a little longer to start up than most
+	  buses, it is very flexible since many different types of storage
+	  device can be attached.
+
+config SPL_USB_HOST
+	bool "Support USB host drivers"
+	depends on SPL
+	help
+	  For detailed help see USB_HOST Kconfig symbol. This option enables
+	  the drivers in drivers/usb/host as part of an SPL build.
 
 config USB_XHCI_HCD
 	bool "xHCI HCD (USB 3.0) support"