mtd: nand: qcom: support for different DEV_CMD register offsets

The FLASH_DEV_CMD registers starting offset is not same in
different QPIC NAND controller versions. This patch adds
the starting offset in NAND controller properties and uses
the same for calculating the actual offset of these registers.

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