arm, imx6: add alternative PAD_CTL_DSE constants

arm, imx6: add alternative PAD_CTL_DSE constants

Not all i.MX6 pads use the same drive strength table. So far only the
240 Ohm to 34 Ohm table was available. Because the constants used have
speaking names it can be confusing to use e.g. PAD_CTL_DSE_48ohm when
according to the reference manual 52 Ohm is the correct value. This
patch adds the 260 Ohm to 37 Ohm table.

For example, the IOMUXC_SW_PAD_CTL_PAD_SD2_CLK register (SD-card clock)
uses the added table.

Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>