mtd: rawnand: loongson: Add nand chip select support

mtd: rawnand: loongson: Add nand chip select support

The page address register describes the page address of the starting
address for NAND read/write/erase operations.

According to the manual, it consists of two parts:
	{chip select, page number}

The `chip select` is fixed at 2 bits, and the `page number` is
determined based on the actual capacity of the single-chip memory.
Therefore we need to determine the `chip select` bits base on the `page
number`.

For example, for a 1GB capacity chip (2K page size), it has 1M pages.
Thus, [19:0] is used to represent the page number, and [21:20]
represents the chip select.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>