ARM: dts: dra7-evm: Fix NAND GPMC timings

The nand timings were scaled down by 2 to account for
the 2x rate returned by clk_get_rate(gpmc_fclk).

As the clock data got fixed by [1], revert back to actual
timings (i.e. scale them up by 2).

Without this NAND doesn't work on dra7-evm.

[1] - commit dd94324b
    ARM: dts: dra7xx-clocks: Fix the l3 and l4 clock rates

Fixes: ff66a3c8 ("ARM: dts: dra7: add support for parallel NAND flash")
Cc: <stable@vger.kernel.org>        [3.16]
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>