clk: aspeed: Fix APLL calculate formula from ast2600-A2

[ Upstream commit 6286ce1e ]

Starting from A2, the A-PLL calculation has changed. Use the
existing formula for A0/A1 and the new formula for A2 onwards.

Fixes: d3d04f6c

 ("clk: Add support for AST2600 SoC")
Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
Link: https://lore.kernel.org/r/20210119061715.6043-1-ryan_chen@aspeedtech.com
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>