[PATCH v1] spi: designware: add support for bits-per-word DT binding

[Ng, Boon Khai]

Hi, Tom, Conor,
>> configuration.
> 
> Thanks. And can we reasonably follow what Linux does here instead?
> 

After study the linux code, the bits_per_word value can be set at the 
higher level stack for example the touch screen driver ad7877.c at line 
https://github.com/torvalds/linux/blob/1f318b96cc84d7c2ab792fcc0bfd42a7ca890681/drivers/input/touchscreen/ad7877.c#L694

will implement something like that in U-Boot too as follow:
1) Add the bits_per_word in the spi_slave struct
2) at the dw_spi_xfer function to resolve for the slave->bits_per_word 
value and store into the priv->bits_per_word, default to 8 otherwise

Regards,
Boon Khai

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