arm: socfpga: fix qspi flash compatible (add "spi-flash")

arm: socfpga: fix qspi flash compatible (add "spi-flash")

This patch adds "spi-flash" to the compatible list of the qspi flash
chip for all socfpga boards. This is required to make qspi work on
these boards on top of the recent fixes. Without the "spi-flash"
compatible string for the flash chip, the speed cannot be read and a
speed of 0Hz is used (which results in a divide-by-zero on these
boards).

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>