Fix EHCI driver by excluding first 512K from available memory

On Zynq 256K-512K memory region is not accessible by all bus masters.
EHCI driver fails when trying to use it for DMA transfers. Patching
memory node does not help because ubldr overrides values there with
the ones obtained from u-boot. So as a workaround we just mark first
512K as reserved.

PR: 211484
Submitted by: Thomas Skibo <thoma555-bsd@yahoo.com>
MFC after: 3 days