mtd: spi-nor-core: Rework s25_mdp_ready() to support Octal DTR mode

s25_mdp_ready() handles status polling for multi-die package parts that
requires to read and check status register for each die. To support
S28HS02GT(dual-die package with Octal DTR support), rename function and
use nor->rdsr_dummy in octal DTR mode.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>