ARM: shmobile: rcar-gen2: Add support for CPG/MSSR bindings

When using the new CPG/MSSR bindings, there is no longer a
"renesas,rcar-gen2-cpg-clocks" node, and the code to obtain the external
clock crystal frequency falls back to a default of 20 MHz.
While this is correct for all upstream R-Car Gen2 and RZ/G1 boards, this
is not necessarily the case for out-of-tree third party boards.

Add support for finding the external clock crystal oscillator on RZ/G1M,
and on R-Car H2, M2-W, and M2-N using the new CPG/MSSR bindings, through
the corresponding "renesas,r8a77xx-cpg-mssr" nodes.

Note that this is not needed on R-Car V2H and E2, and on RZ/G1E, as on
those SoCs the arch_timer and generic counter clock is derived from the
ZS clock instead.

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