uboot-imx

dm: core: Add functions to get strings and the string count from a stringlist

dev_read_string_count() is used to get the number of strings in a
stringlist.
dev_read_string_index() is used to get a string in the stringlist based on
its position in the list.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>