mtd: nand: qcom: reorganize nand devices probing

The NAND controller can support multiple NAND devices having different
page sizes. Future code will require us to allocate memory based on the
maximum number of codewords among all the devices. We reorganize the
NAND device probing such that the ONFI parameters are first read for
each connected device to identify the maximum number of codewords
possible, and only then proceed with MTD device registration (i.e, call
nand_scan_tail and mtd_device_register).

This is a reorganization of the existing code and will not change any
functionality.

Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>