mtd: spi-nor: add a quad_enable callback in struct flash_info

Some manufacturers may use different bit to set QE on different
memories.

The GD25Q256 from GigaDevice is an example, which uses S6(bit 6
of the Status Register-1) to set QE, which is different with
other supported memories from GigaDevice that use S9(bit 1 of
the Status Register-2). This makes it is impossible to select
the quad enable method by distinguishing the MFR. This patch
introduce a quad_enable function which can be set per memory
in the flash_info list table.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>