arm: kirkwood: 88f6281: Detect CONFIG_SYS_TCLK from SAR register

Bit 21 in SAR register specifies if TCLK is running at 166 MHz or 200 MHz.
This information is undocumented in public Marvell Kirkwood Functional
Specifications [2], but is available in Linux v3.15 kirkwood code [1].

Commit 8ac303d4 ("arm: kirkwood: Do not overwrite CONFIG_SYS_TCLK")
broke support for Marvell 88F6281 SoCs because it was expected that all
those SoCs have TCLK running at 200 MHz as specified in Marvell 88F6281
Hardware Specifications [3].

Fix broken support for 88F6281 by detecting CONFIG_SYS_TCLK from SAR
register, like it was doing Linux v3.15.

[1] - https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/arm/mach-kirkwood/common.c?h=v3.15#n542
[2] - https://web.archive.org/web/20130730091033/http://www.marvell.com/embedded-processors/kirkwood/assets/FS_88F6180_9x_6281_OpenSource.pdf
[3] - https://web.archive.org/web/20120620073511/http://www.marvell.com/embedded-processors/kirkwood/assets/HW_88F6281_OpenSource.pdf



Update by Stefan 2022-08-23:
- Fix compilation error for ds109

Fixes: 8ac303d4 ("arm: kirkwood: Do not overwrite CONFIG_SYS_TCLK")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>