ARMv8: fix bug for flush data cache by set/way

When flush the d$ with set/way instruction, it need calculate the way's
offset = log2(Associativity); but in current uboot's code, it use below
formula to calculate the offset: log2(Associativity * 2 - 1), so finally
it cannot flush data cache properly.

Signed-off-by: Leo Yan <leoy@marvell.com>