MLK-10215 Add elan init in i.MX6SL-EVK boardl5.0.0_1.0.0-beta

EPDC board contain a elan touch screen, this screen is a i2c
slave. If this EPDC board connect to i.MX6SL-EVK board, after
uboot boot up, if we do i2c operation, like i2c probe, then
the i2c bus block. This is due to the elan touch screen i2c slave.
This device needs to do some initialization opearation before its
i2c operation, otherwise this i2c device pull down the i2c clk line,
and make the i2c bus hang. This means elan needs a special flow on
i2c before its address is acked, otherwise the i2c bus will be hang.

This patch is a workaround, it add a void function which is defined
as a weak symbol in i2c driver, and it is called before every i2c
operation. In mx6slevk, this function was overwrite to execute elan
initialization. So that, for mx6slevk board, it will  initialize
elan before every i2c operation, but for other boards, it just work
as before.

Signed-off-by: Haibo Chen <haibo.chen@freescale.com>

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