leds: lm3601x: Move driver to flash subdirectory

We created a subdirectory for LED drivers that depend on
CONFIG_LEDS_CLASS_FLASH, and this driver does so let's
move it there.

Cc: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>