pinmux_dev: Remove PINMUX_SELECT_REGISTER from quark_mcu driver

This patch removes the PINMUX_SELECT_REGISTER macro definition from
pinmux_dev_quark_mcu.c since it is already defined in pinmux_quark_mcu.h
(which is included by the .c file).

Change-Id: I468cf6a54fc30d681f42a59eb2c8401d2b180849
Signed-off-by: Andre Guedes <andre.guedes@intel.com>