spl: add option for adding post memory test to the SPL framework

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Cc: Sergei Shtylyov <sshtylyov@mvista.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>