The Zynq has a bug where the L2 cache will return invalid data in some
circumstances unless the L2C_RAM register is set to 0x00020202 before the first
enabling of the L2 cache.

The Xilinx-recommended solution to this problem is to ensure that early one of
the earlier bootstages correctly initialize L2C_RAM, however, this issue wasn't
discovered and fixed until after their EDK/SDK 14.4 release.  For systems built
prior to that, and which lack field-upgradable bootloaders, this issue still
exists and silent data corruption can be seen in the wild.

Fix these systems by ensuring L2C_RAM is properly initialized at the
earliest convenient moment prior to the L2 being brought up, which is
when the SLCR is first mapped.

The Zynq bug is described in more detail by Xilinx AR# 54190 as quoted
below.

Xilinx AR# 54190
http://www.xilinx.com/support/answers/54190.htm
Captured on 2014-09-24 14:43 -0500

  = Description =
  For proper L2 cache operation, the user code must program the
  slcr.L2C_RAM register (address 0xF800_0A1C) to the value of
  0x0002_0202 before enabling the L2 cache. The reset value
  (0x0001_0101) might cause, very infrequently, the L2 cache to return
  invalid data.

  = Solution =
  It is up to the user code (FSBL or other user code) to set the
  slcr.L2C_RAM register to the value 0x0002_0202 before enabling the L2
  cache.

  Note: The L2 cache is disabled after reset and is not enabled by the
  BootROM.

  Note: The slcr.l2C_RAM register was previously reserved. It is added
  in the Zynq-7000 AP SoC Technical Reference Manual (TRM) v1.5 as
  "Reserved".

Thanks to Jaeden Amero for initial debugging and triage efforts.
Signed-off-by: Josh Cartwright <joshc@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>