ddr: marvell: a38x: Fix Synchronous vs Asynchronous mode determination

ddr: marvell: a38x: Fix Synchronous vs Asynchronous mode determination

Before commit 4c289425752f ("mv_ddr: a38x: add support for ddr async
mode"), Asynchornous Mode was only used when the CPU Subsystem Clock
Options[4:0] field in the SAR1 register was set to value 0x13: CPU at
2 GHz and DDR at 933 MHz.

Then commit 4c289425752f ("mv_ddr: a38x: add support for ddr async
mode") added support for Asynchornous Modes with frequencies other than
933 MHz (but at least 467 MHz), but the code it added to check for
whether Asynchornous Mode should be used is wrong: it checks whether the
frequency setting in board DDR topology map is set to value other than
MV_DDR_FREQ_SAR.

Thus boards which define a specific value, greater than 400 MHz, for DDR
frequency in their board topology (e.g. Turris Omnia defines
MV_DDR_FREQ_800), are incorrectly put into Asynchornous Mode after that
commit.

The A38x Functional Specification, section 10.12 DRAM Clocking, says:
  In Synchornous mode, the DRAM and CPU clocks are edge aligned and run
  in 1:2 or 1:3 CPU to DRAM frequency ratios.

Change the check for whether Asynchornous Mode should be used according
to this explanation in Functional Specification.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>