phy: PHY_FSL_LYNX_28G should depend on ARCH_LAYERSCAPE

Freescale Layerscape Lynx 28G SerDes PHYs are only present on
Freescale/NXP Layerscape SoCs.

Move PHY_FSL_LYNX_28G outside the block for ARCH_MXC, as the latter
is meant for i.MX8 SoCs, which is a different family than Layerscape.
Add a dependency on ARCH_LAYERSCAPE, to prevent asking the user about
this driver when configuring a kernel without Layerscape SoC support.

Fixes: 02e2af20 ("Merge tag 'char-misc-5.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc")
Fixes: 8f73b37c

 ("phy: add support for the Layerscape SerDes 28G")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>