memory: omap-gpmc: Correct kerneldoc

Use proper kerneldoc to fix GCC warnings like:

    drivers/memory/omap-gpmc.c:299: warning: Function parameter or member 'cs' not described in 'gpmc_get_clk_period'
    drivers/memory/omap-gpmc.c:432: warning: Excess function parameter 'ma' description in 'get_gpmc_timing_reg'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>