spi: remove forgotten CONFIG_SPI_BCM53XX

I accidentally sent an early version of patch removing spi-bcm53xx
driver which got rid of .c and .h files *only*. I amended local commit
but forgot to re-format the patch.

This commit removes leftovers of dropped driver.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Mark Brown <broonie@kernel.org>