soc: imx8m: Remove global soc_uid

The static global soc_uid is only ever used as kasprintf() parameter in
imx8m_soc_probe(). Pass pointer to local u64 variable to .soc_revision()
callback instead and let the .soc_revision() callback fill in the content.
Remove the unnecessary static global variable.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>