spi: octeon_spi: Use a fixed 100MHz input clock on Octeon TX2

spi: octeon_spi: Use a fixed 100MHz input clock on Octeon TX2

Octeon TX2 sets the TB100_EN bit in the config register. We need to use
a fixed 100MHz clock for this as well to work properly.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Suneel Garapati <sgarapati@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>