ARM: davinci: mark spi_board_info arguments as const

spi_board_info is not modified in any of the spi registeration
functions and passed along as-is to the generic SPI layer.

Mark spi_board_info arguments as const to signal this.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[nsekhar@ti.com: extended for dm365 and dm355]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>