arch: arm: arm_mpu_v7m: Fix unsupported Cortex-R access permission mode

This file previously defined an MPU access permission mode of 0x7 which
corresponded to privileged read-only, unprivileged read-only, similar to
mode 0x6. However, it appears that at least Cortex-R5 does not support
this mode, defining 0x7 as UNP (Unpredictable) or a value which should
not be used.

This value was in turn referenced by the REGION_FLASH_ATTR macro, which
caused the offending value to be used when a memory region was declared
as DT_MEM_ARM(ATTR_MPU_FLASH) in the device tree, causing such regions
to not work properly on Cortex-R5.

Since 0x6 is supported by both Cortex-M and Cortex-R and does the same
thing, there is no reason to use 0x7. Remove the RO_Msk definition which
referenced it, and change REGION_FLASH_ATTR to use P_RO_U_RO_Msk instead.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>