leds: sgm3140: 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: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>