arm64: zynqmp: Fix User MTD partition size

The commit c8630167 ("arm64: zynqmp: Add mtd partition for secure OS
storage area") didn't update User partition size that's why size was beyond
actual device size.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/0a56405553b87a75e066cd71697cafe7c1c97eef.1681309812.git.michal.simek@amd.com