riscv: sifive/fu540: spl: Rename soc_spl_init()

spl_soc_init() seems to be a better name, as all SPL functions
names start from the spl_ prefix.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Pragnesh Patel <pragnesh.patel@sifive.com>