spi: remove the older/duplicated bcm53xx driver

This driver was added by commit 0fc6a32 ("spi: bcm53xx: driver for
SPI controller on Broadcom bcma SoC") back in 2014. It was needed to
provide a minimal support for SPI controller on BCM5301X (AKA Northstar)
devices.

An alternative driver was added by Kamal in commit fa236a7 ("spi:
bcm-qspi: Add Broadcom MSPI driver") 2 years later. It supports the same
hardware but for some reason a new driver has been developed for it.

At this point the new driver supports: more modes, setting a speed,
setting bits per word and uses IRQs instead of polling. DTS file for
BCM5301X has also been updated in the commit 1c8f406 ("ARM: dts:
BCM5301X: convert to iProc QSPI") - over a year ago.

That explained I see no reason to keep the old driver alive.

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