arm: socfpga: make SPL_TEXT_BASE overridable

To boot from fpga on socfpga gen5, we need to set CONFIG_SPL_TEXT_BASE to
0xC0000000 (hps2fpgaslaves base address).

Since converting CONFIG_SPL_TEXT_BASE to Kconfig hasn't been successful so
far, let's make this value overridable in socfpga_common.h, so that we can
have different board configs override this in socfpga_common.h.

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