arm,imx6: fix PAD_CTL_SPEED_LOW constant

For most i.MX6 processors the PAD_CTL_SPEED_LOW constant is the same.
Only the i.MX6 SoloLite is an exemption. So far the code did not
consider that. Additionally, for a few i.MX6 processors the code used
the wrong value for the constant.

This patch fixes the PAD_CTL_SPEED_LOW constant for:
 - i.MX6 Solo [1]
 - i.MX6 DualLite [1]
 - i.MX6 Dual [2]
 - i.MX6 Quad [2]
 - i.MX6 DualPlus [3]
 - i.MX6 QuadPlus [3]

Before, it was already correct for:
 - i.MX6 SoloLite [4]
 - i.MX6 SoloX [5]
 - i.MX6 UtraLite [6]
 - i.MX6 ULL [7]

[1] https://www.nxp.com/docs/en/reference-manual/IMX6SDLRM.pdf
[2] https://www.nxp.com/docs/en/reference-manual/IMX6DQRM.pdf
[3] https://www.nxp.com/docs/en/reference-manual/iMX6DQPRM.pdf
[4] https://www.nxp.com/docs/en/reference-manual/IMX6SLRM.pdf
[5] https://www.nxp.com/docs/en/reference-manual/IMX6SXRM.pdf
[6] https://www.nxp.com/docs/en/reference-manual/IMX6ULRM.pdf
[7] https://www.nxp.com/docs/en/reference-manual/IMX6ULLRM.pdf



Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Reviewed-by: Heiko Schocher <hs@denx.de>