ARM: dts: socfpga: memreserve first 4KB for future system use

This patch adds a /memreserve/ section to reserve the first 4K for future
use by the system. One possible use-case is trampoline code used to bring
secondary cores online.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Pavel Machek <pavel@denx.de>
---
v3: Update commit message based on Mark Rutland's comment
v2: Add a comment in the dts files