imx8m: set sane default value for SPL_LOAD_FIT_ADDRESS

imx8m: set sane default value for SPL_LOAD_FIT_ADDRESS

I enabled IMX_HAB on an imx8mp board, but even though I knew about the
implementation, I forgot that I had to provide a sane value for
SPL_LOAD_FIT_ADDRESS. The help text for IMX_HAB doesn't mention this
implicit requirement, and there's no build-time warning; the default
0x0 value just ends up being returned from
board_spl_fit_buffer_addr(), obviously resulting in a non-booting
board.

The existing imx8m* board configs that set a non-zero value currently
all use 0x44000000. The actual value doesn't matter too much, but 0 is
always wrong for imx8m platforms. So just use 0x44000000 as default
for those platforms.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>