mtd: lpddr_cmds: make function static

do_xxlock() is only used locally. This silences a sparse warning:

drivers/mtd/lpddr/lpddr_cmds.c:706:5: warning: no previous prototype for 'do_xxlock' [-Wmissing-prototypes]

Signed-off-by: Brian Norris <computersforpeace@gmail.com>