integrator: do not test first part of the memory

When booting from Flash, the Integrator remaps its flash memory
from 0x24000000 to 0x00000000, and starts executing it at
0x00000000. This ROM thus hides the RAM underneath and first
0x40000 bytes of the memory cannot be tested by get_ram_size().
So let's test from 0x40000 to the end of detected memory
instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>