arm: xilinx_zynq: Add linux pre-boot

arm: xilinx_zynq: Add linux pre-boot

Add a Linux-specific pre-boot routine that matches the device-
specific bootloaders behaviour. This is needed for modern Linux that
expects the ARM PLL in SLCR to be a more even value (not 26).

Cc: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 9a9025ea65572586b50dca4e5819032e3c436d64.1446182614.git.crosthwaite.peter@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>