board: sifive: Rename spl_soc_init() to spl_dram_init()

Rename spl_soc_init() to spl_dram_init() because the generic function
name does not reflect what the function actually does. Also
spl_dram_init() is commonly used for dram initialization and should be
called from board_init_f().

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>