omap3 flash: rename board-sdp-flash.c to be use by other boards

rename board-sdp-flash.c(board-flash.c) and board-sdp.h(board-flash.h) to
used by other board e.g. zoom

Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>