spi: spi-gpio: Implement spidelay when requested bit rate <= 1 Mbps

Formerly the delay was omitted as bit-banged SPI seldom achieved
even one Mbit/s; but some modern platforms can run faster, and
some SPI devices may need to be clocked slower.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>