mtd: spi-nor-core: Rework spi_nor_cypress_octal_dtr_enable()

Enabling Octal DTR mode in multi-die package parts requires reister setup
for each die. That can be done by simple for-loop. write_enable() takes
effect to all die at once so we can call it before the loop. Besides we
can replace spi_mem_exec_op() calls with spansion_read/write_any_reg().
And finally, we must mask CFR2V[7:4] when changing dummy cycles, as
CFR2V[7] indicates current addressing mode and that should be 1 (4-byte
address mode) for multi-die package parts.

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