mtd: spi-nor: Use spi-mem dirmap API

This adds support for the dirmap API to the spi-nor subsystem, as
introduced in Linux commit df5c21002cf4  ("mtd: spi-nor: use
spi-mem dirmap API").

This patch is synchronize from the following patch
https://patchwork.ozlabs.org/project/uboot/patch/20210205043924.149504-4-seanga2@gmail.com/


The corresponding Linux kernel SHA1 is df5c21002cf4.

Signed-off-by: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Acked-by: Pratyush Yadav <p.yadav@ti.com>