All of the instances of imx*_boot_save_loc() do exactly the same thing
with the only difference being SoC-specific imx*_get_boot_source
call. Convert the code into a generic function taking function pointer
to perform SoC specific bits.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>