spi: designware: convert to livetree

Convert 'dw_spi_ofdata_to_platdata' to use 'dev_read_u32_default'
instead of 'fdtdec_get_int' and get rid of DECLARE_GLOBAL_DATA_PTR.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>