i.MX6: add enable_spi_clk()

add enable_spi_clk(), so board code can enable spi clocks.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Stefano Babic <sbabic@denx.de>