loader: Fix incorrect parameter name in load_image_mr() macro

loader: Fix incorrect parameter name in load_image_mr() macro

Fix a typo in the load_image_mr() macro: 'mr' was written when
the parameter name is '_mr'. (This had no visible effects since
the single use of the macro used 'mr' as the argument.)

Fixes 76151cacfe956248a25b38b5e8429465584f47bb "loader: Add
load_image_mr() to load ROM image to a MemoryRegion"

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>