spi: bcm63xx-hsspi: Add prepend mode support

spi: bcm63xx-hsspi: Add prepend mode support

Due to the controller limitation to keep the chip select low during the
bus idle time between the transfer, a dummy cs workaround was used when
this driver was first upstreamed to the u-boot based on linux kernel
driver. It basically picks the dummy cs as !actual_cs so typically dummy
cs is 1 when most of the case only cs 0 is used in the board design.
Then invert the polarity of both cs and tell the controller to start the
transfers using dummy cs. Assuming both cs are active low before the
inversion, effectively this keeps dummy cs high and actual cs low during
the transfer and workaround the issue.

This workaround requires that dummy cs 1 pin to is set to SPI chip
selection function in the pinmux when the transfer clock is above 25MHz.
The old chips likely have default pinmux set to chip select on the dummy
cs pin so it works but this is not case for the new Broadband BCA chips
and this workaround stop working. This is specifically an issue to
support SPI NAND and SPI NOR flash because these flash devices can
typically run at or above 100MHz.

This patch utilizes the prepend feature of the controller to combine the
multiple transfers in the same message to a single transfer when
possible. This way there is no need to keep clock low between transfers
and solve the issue without any pinmux requirement.

Multiple transfers within a SPI message may be combined into one
transfer if the following are all true:
  * One or more half duplex write transfer in single bit mode
  * Optional full duplex read/write at the end
  * No delay and cs_change between transfers

Most of the SPI device meets this requirements such as SPI NOR, SPI NAND
flash, Broadcom SPI voice card and etc. So this change switches to the
prepend mode as the default mode. For any SPI message that does not meet
the above requirement, we switch to original dummy cs mode but limit the
clock rate to the safe 25MHz.

Port from linux patch:
Link: https://lore.kernel.org/r/20230209200246.141520-12-william.zhang@broadcom.com
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>