ARM: imx6: mark OCRAM mapping read-only

iMX6 needs to write some code to OCRAM which resumes the DDR controller
after suspend. However, merely using __arm_ioremap_exec() causes the
kernel to complain of a W+X mapping. Solve this by using the newly
introduced __arm_iomem_set_ro() function to prevent inadvertent or
malicious writes to code we may later execute.

Tested-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>