xilinx_timer: Fix writes into TCSR register

The TCSR register has only 11 valid bits. This is now used by the
linux kernel to auto-detect endianness, and causes Linux 3.15-rc1
and later to hang when run under qemu-microblaze. Mask valid bits
before writing the register to solve the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
