omap3: SDP: Introducing 'board-sdp-flash.c' for flash init

This patch adds 'board-sdp-flash.c', which could be utilized
by boards similar to 3430SDP. (For ex: 2430sdp, 36030sdp).

This file does initialization for all three flash devices present
in SDP boards (NOR, NAND, OneNAND), by finding there 'cs' number
dynamically using switch setting information (S8: 1-4).
This also expects partition information from core board files (for
ex: board-3430sdp.c). Which allows to choose different default
partitions for different boards.

A new structure is created for this purpose: 'flash_partitions'
in 'mach/board-sdp.h'. This has two members:
1. struct mtd_partition *parts
2. int nr_parts

A board file is expected to fill this structure and pass it to
'sdp-flsash-init'. Partition information should be passed in
structure array of 'flash_partitions'. Partition information should
be passed in below sequence in array:
NOR
OneNAND
NAND

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>