spmi: pmic_arb: make selectable on ARCH_QCOM

With the split of Qualcomm MSM support into legacy and multiplatform,
the SPMI PMIC arb driver is only relevant on the multiplatform supported
SoCs. Switch the Kconfig depends to ARCH_QCOM.

Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>