ARM: imx31: Fix IIM mapping leak in revision check

ARM: imx31: Fix IIM mapping leak in revision check

mx31_read_cpu_rev() maps the IIM registers with of_iomap() to read the
silicon revision, but returns without unmapping the MMIO mapping.

Keep the normalized revision value in a local variable and route the
return path through iounmap() after the revision register has been read.

Fixes: 3172225d45bd ("ARM: imx31: Retrieve the IIM base address from devicetree")
Signed-off-by: Yuho Choi <dbgh9129@gmail.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>