mtd: nand: mxs_nand_spl: add mxs_flash_full_ident

For now, the existing SPL MXS NAND driver only supports to identify
ONFi-compliant NAND chips. In order to allow identifying
non-ONFi-compliant chips add `mxs_flash_full_ident()` which uses the
`nand_get_flash_type()` functionality from `nand_base.c` to lookup
for supported NAND chips in the chip ID list.

For compatibility reason the full identification support is only
available if the config option `CONFIG_SPL_NAND_IDENT` is enabled.

The lookup was tested on a custom i.MX6ULL board with a Toshiba
TC58NVG1S3HTAI0 NAND chip.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>