i2c: I2C_MLXCPLD on ARM64 should depend on ACPI

The "i2c_mlxcpld" platform device is only instantiated on X86 systems
(through drivers/platform/x86/mlx-platform.c), or on ARM64 systems with
ACPI (through drivers/platform/mellanox/nvsw-sn2201.c).  Hence further
restrict the dependency on ARM64 to ACPI, to prevent asking the user
about this driver when configuring an ARM64 kernel without ACPI support.

While at it, document in the Kconfig help text that the driver supports
ARM64/ACPI based systems, too.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Vadim Pasternak <vadimp@nvidia.com>
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>