ARM: shmobile: r8a7791: Use common R-Car Gen2 machine definition

Now r8a7791 no longer needs the SMP initialization fallback, it can use
the common R-Car Gen2 machine definition, and the r8a7791-specific one
can be removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>