memory: omap-gpmc: Fix build error without CONFIG_OF

memory: omap-gpmc: Fix build error without CONFIG_OF

If CONFIG_OF is n, gcc fails:

drivers/memory/omap-gpmc.o: In function `gpmc_omap_onenand_set_timings':
    omap-gpmc.c:(.text+0x2a88): undefined reference to `gpmc_read_settings_dt'

Add gpmc_read_settings_dt() helper function, which zero the gpmc_settings
so the caller doesn't proceed with random/invalid settings.

Fixes: a758f50f10cf ("mtd: onenand: omap2: Configure driver from DT")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Link: https://lore.kernel.org/r/20200827125316.20780-1-yuehaibing@huawei.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>