arm64: dts: uniphier: add reserved-memory for secure memory

The memory regions specified by /memreserve/ are passed to
early_init_dt_reserve_memory_arch() with nomap=false, so it is
not suitable for reserving memory for Trusted Firmware-A etc.

Use the more robust /reserved-memory node with the no-map property
to prevent the kernel from mapping it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>