dmaengine: shdmac: Change platform check to CONFIG_ARCH_RENESAS

Since commit 9b5ba0df ("ARM: shmobile: Introduce ARCH_RENESAS")
is CONFIG_ARCH_RENESAS a more appropriate platform check than the legacy
CONFIG_ARCH_SHMOBILE, hence use the former.

Renesas SuperH SH-Mobile SoCs are still covered by the CONFIG_CPU_SH4
check, just like before support for Renesas ARM SoCs was added.

Instead of blindly changing all the #ifdefs, switch the main code block
in sh_dmae_probe() to IS_ENABLED(), as this allows to remove all the
remaining #ifdefs.

This will allow to drop ARCH_SHMOBILE on ARM in the near future.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>