spi: cadence_qspi: convert to livetree

Convert 'cadence_spi_ofdata_to_platdata' to use dev_read_* functions to
read driver parameters and 'dev_read_first_subnode'/'ofnode_read_*' to
read flash (child node) parameters.

Tested on socfpga_socrates (socfpga gen5).

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