tcl/target/stm32l4x: set default WORKAREASIZE to smallest device

Change-Id: Ia8bfb664ff28bd0579492032ce513b010e71c593
Signed-off-by: Markus Reiter <me@reitermark.us>
Reviewed-on: https://review.openocd.org/c/openocd/+/6931


Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>