board: .gitignore: replace dsdt.c by dsdt_generated.c

Since commit 5d94cbd1 ("scripts: Makefile.lib: generate
dsdt_generated.c instead of dsdt.c"), the file generated
is named dsdt_generated.c instead of dsdt.c.
So all files .gitignore referencing dsdt.c should be
upated with dsdt_generated.c.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>