soc: fsl: guts: embed fsl_guts_get_svr() in probe()

Move the reading of the SVR into the probe function as
fsl_guts_get_svr() is the only user of the static guts variable and this
lets us drop that as well as the malloc() for this variable. Also, we
can unmap the memory region after we accessed it, which will simplify
error handling later.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>