MLK-19452-2 ARM: imx_v7_defconfig: Add spi slave mode

MLK-19452-2 ARM: imx_v7_defconfig: Add spi slave mode

Add spi slave mode and enable spi slave time/system control
functions.

Example for SPI slave time:
SLAVE:  # echo spi-slave-time > /sys/class/spi_slave/spi0/slave
MASTER: # spidev_test -D /dev/spidev0.0 -p dummy-8B -v
    spi mode: 0x0
    bits per word: 8
    max speed: 500000 Hz (500 KHz)
    RX | 00 00 04 6D 00 09 5B BB __ __ __ __ __ __ __ _...
	       ^^^^^    ^^^^^^^^
	       seconds  microseconds

Example for SPI slave system control:
SLAVE:  # echo spi-slave-system-control >
			    /sys/class/spi_slave/spi0/slave
BOTH:   # reboot='\x7c\x50'
BOTH:   # poweroff='\x71\x3f'
BOTH:   # halt='\x38\x76'
BOTH:   # suspend='\x1b\x1b'
MASTER: # spidev_test -D /dev/spidev0.0 -p $suspend
			    or $reboot, $poweroff, $halt

Signed-off-by: Xiaoning Wang <xiaoning.wang@nxp.com>
Signed-off-by: Vipul Kumar <vipul_kumar@mentor.com>