mtd: spi-nor: Rename s25hx_t prefix

Rename s25hx_t prefix to s25 so that the single set of fixup hooks can
support all other S25 families.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Acked-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>