i.MX6QP revision 2.1 currently are not recognized, causing
imx_set_silicon_revision to print IMX_CHIP_REV_UNKNOWN as 15.15:

  detected i.MX6 Quad Plus revision 15.15

To fix this, instead of extending the list with yet another variant,
switch to the method introduced in the Linux kernel in commit
c22dee6e3c16 ("ARM: imx: Improve the soc revision calculation flow").

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>