spl: move SPL driver entries to driver/Makefile

Just preparing for upcoming cleaning.

The board-specific linker script  board/vpac270/u-boot-spl.lds
has been touched to avoid build error.  It does not change the
size of spl/u-boot-spl.bin for this board, so it should be OK.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>