perf: MARVELL_CN10K_DDR_PMU should depend on ARCH_THUNDER

perf: MARVELL_CN10K_DDR_PMU should depend on ARCH_THUNDER

The Marvell CN10K DRAM Subsystem (DSS) performance monitor is only
present on Marvell CN10K SoCs.  Hence add a dependency on ARCH_THUNDER,
to prevent asking the user about this driver when configuring a kernel
without Cavium Thunder (incl. Marvell CN10K) SoC support,

Fixes: 68fa55f0e05c ("perf/marvell: cn10k DDR perf event core ownership")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/18bfd6e1bcf67db7ea656d684a8bbb68261eeb54.1648559364.git.geert+renesas@glider.be
Signed-off-by: Will Deacon <will@kernel.org>