arm: socfpga: Zap spl.h and ad-hoc related syms

Switch to the common spl.h file and zap the arch/spl.h . Since the arch/spl.h
contained various ad-hoc symbols, zap those symbols as well and rework the
board configuration a little so it doesn't depend on them.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Vince Bridgers <vbridger@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Acked-by: Pavel Machek <pavel@denx.de>