mtd: denali: use CONFIG_SYS_NAND_SELF_INIT

Some variants of the Denali NAND controller need some registers
set up based on the device information that has been detected during
nand_scan_ident().

CONFIG_SYS_NAND_SELF_INIT has to be defined to insert code between
nand_scan_ident() and nand_scan_tail().  It is also helpful to reduce
the difference between this driver and its Linux counterpart because
this driver was ported from Linux.  Moreover, doc/README.nand recommends
to use CONFIG_SYS_NAND_SELF_INIT.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Chin Liang See <clsee@altera.com>