mtd: Add helper macro for register_mtd_blktrans boilerplate

This patch introduces the module_mtd_blktrans macro which is a convenience
macro for mtd blktrans modules similar to module_platform_driver.
It is intended to be used by drivers which init/exit section does nothing
but register/unregister the mtd blktrans driver. By using this macro it is
possible to eliminate a few lines of boilerplate code per mtd blktrans
driver.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210213164600.409061-2-zhengdejin5@gmail.com