ARM: shmobile: r8a7794: IPMMU compat string SoC part number update

Update IPMMU compat strings to include SoC part number.

By specifying SoC part number in DT it becomes possible
to implement SoC specific features in the IPMMU driver.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>