ARM: zynq: Fix bootmode mask

Bootmode mask was defined as 0x0F, but documentation mentions 0x07.
Experiments show that bit "3" is the JTAG chain configuration.
Change the mask to "7" to allow systems with a different chain
configuration to boot correctly.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Acked-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>