spi: davinci: remove unnecessary private data member 'region_size'

Remove unnecesary private data member 'region_size'
being used to store the size of SPI memory region.

Instead, get the memory resource size directly from
the platform data.

Tested-By: Brian Niebuhr <bniebuhr@efjohnson.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>