n1sdp/n1sdp_flash: add new module n1sdp_flash

This patch adds new n1sdp_flash module under product/n1sdp/module/
directory.

N1SDP board has external QSPI flash memory in which the firmware
image containing SCP BL2, MCP BL2, AP-BL31, AP-BL33 & NVM
configuration data will be placed in N1SDP specific customized
layout.

The n1sdp_flash module is responsible for parsing the customized
layout, identifying the images and saving the image descriptors in
its context. Other modules can bind to n1sdp_flash to retrieve this
information.

Change-Id: I29643045b1cb91ec37f4772183f40e269714ac5a
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>