imx: make 5.15 default kernel and remove 5.10 support

imx: make 5.15 default kernel and remove 5.10 support

In imx target we're sharing single, version agnostic kernel
`config-default` file, which doesn't work very well with current 5.10
and upcoming 5.15 kernel symbols as recent rebase onto 5.15 kernel
introduced in commit 2b395c298247 ("imx: update config for 5.15) has
introduced following regression with 5.10 kernel:

 Marvell 88E6xxx Ethernet switch fabric support (NET_DSA_MV88E6XXX) [Y/n/m/?] y
   Switch Global 2 Registers support (NET_DSA_MV88E6XXX_GLOBAL2) [Y/n/?] (NEW)

That NET_DSA_MV88E6XXX_GLOBAL2 kernel config symbol has been removed in
upstream commit 63368a7416df ("net: dsa: mv88e6xxx: Make global2 support
mandatory") in kernel version 5.12.

This issue could be probably fixed by introduction of separate kernel
config files for each currently used kernel versions and subtarget, but
it is not worth the hassle and resources as imx target is running mostly
upstream kernel, so lets fix it by switching to 5.15 version instead.

Fixes: 2b395c298247 ("imx: update config for 5.15")
Acked-by: Piotr Dymacz <pepe2k@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>