usb: gadget: add SDP driver

Add SDP (Serial Downloader Protocol) implementation for U-Boot. The
protocol is used in NXP SoC's boot ROM and allows to download program
images. Beside that, it can also be used to read/write registers and
download complete Device Configuration Data (DCD) sets. This basic
implementation supports downloading images with the imx header format
reading and writing registers.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>