arm: mx6: Reduce SPL malloc pool size

Using 50 MiB malloc pool in SPL is nonsense. Since the caches are not
enabled in SPL, it takes 2 seconds to init the pool and has no obvious
benefit. Reduce the size to 1 MiB.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Tested-by: Stefano Babic <sbabic@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>