clk: ast2600: Fix AHB clock divider for A1

The latest specs for the AST2600 A1 chip include some different bit
definitions for calculating the AHB clock divider. Implement these in
order to get the correct AHB clock value in Linux.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lkml.kernel.org/r/20200408203616.4031-1-eajames@linux.ibm.com
Fixes: d3d04f6c

 ("clk: Add support for AST2600 SoC")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>