mtd: nand: mxs_nand_spl: refactor mxs_flash_ident

The existing `mxs_flash_ident()` is limited to identify ONFi compliant
NAND chips only. In order to support non-ONFi NAND chips refactor the
function and rename it to `mxs_flash_onfi_ident()`.

A follow-up patch will add `mxs_flash_full_ident()` which allows to use
the chip ID list to lookup for supported NAND flashs.

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