ARM: shmobile: porter: add QSPI DT support

Define the Porter board dependent part of the QSPI device node.
Add device nodes  for Spansion  S25FL512S SPI flash and the MTD partitions
on it.

This patch is  mostly analogous  to the commit f59838d4 ("ARM:
shmobile: henninger: add QSPI DT support")  as there are no differences
between the boards in this respect.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>