mtd: lpddr: Mark functions as static and remove unused function

This patch marks the functions do_write_buffer() and do_erase_oneblock()
as static because because they are not used outside this file. It also
removes the unused function word_program() in lpddr/lpddr_cmds.c.

Thus, it also removes the following warnings in lpddr/lpddr_cmds.c:
drivers/mtd/lpddr/lpddr_cmds.c:391:5: warning: no previous prototype for ‘do_write_buffer’ [-Wmissing-prototypes]
drivers/mtd/lpddr/lpddr_cmds.c:472:5: warning: no previous prototype for ‘do_erase_oneblock’ [-Wmissing-prototypes]
drivers/mtd/lpddr/lpddr_cmds.c:751:5: warning: no previous prototype for ‘word_program’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>