ppc/85xx: Fix compiler warning in nand_spl/.../p1_p2_rdb/nand_boot.c

nand_boot.c: In function 'board_init_f':
nand_boot.c:44: warning: 'sys_clk' may be used uninitialized in this function

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>