i2c: Fill req_seq in i2c_post_bind()

For i2c controllers which are missing alias in DT there is no req_seq
setup. This function is setting up proper ID based on highest found
alias ID.

On zcu102 this is the behavior when patch is applied.
ZynqMP> i2c bus
Bus 0:	i2c@ff020000
   20: gpio@20, offset len 1, flags 0
   21: gpio@21, offset len 1, flags 0
   75: i2c-mux@75, offset len 1, flags 0
Bus 2:	i2c@0
Bus 3:	i2c@1
Bus 4:	i2c@2
Bus 1:	i2c@ff030000  (active 1)
   74: i2c-mux@74, offset len 1, flags 0
   75: i2c-mux@75, offset len 1, flags 0
Bus 5:	i2c@0  (active 5)
   54: eeprom@54, offset len 1, flags 0
Bus 6:	i2c@1
Bus 7:	i2c@2
Bus 8:	i2c@3
Bus 9:	i2c@4
Bus 10:	i2c@0
Bus 11:	i2c@1
Bus 12:	i2c@2
Bus 13:	i2c@3
Bus 14:	i2c@4
Bus 15:	i2c@5
Bus 16:	i2c@6
Bus 17:	i2c@7

Before this patch applied (controllers have -1 ID)
ZynqMP> i2c bus
Bus 0:	i2c@ff020000
   20: gpio@20, offset len 1, flags 0
   21: gpio@21, offset len 1, flags 0
   75: i2c-mux@75, offset len 1, flags 0
Bus -1:	i2c@0
Bus -1:	i2c@1
Bus -1:	i2c@2
Bus 1:	i2c@ff030000  (active 1)
   74: i2c-mux@74, offset len 1, flags 0
   75: i2c-mux@75, offset len 1, flags 0
Bus -1:	i2c@0  (active 0)
   54: eeprom@54, offset len 1, flags 0
Bus -1:	i2c@1
Bus -1:	i2c@2
Bus -1:	i2c@3
Bus -1:	i2c@4
Bus -1:	i2c@0
Bus -1:	i2c@1
Bus -1:	i2c@2
Bus -1:	i2c@3
Bus -1:	i2c@4
Bus -1:	i2c@5
Bus -1:	i2c@6
Bus -1:	i2c@7

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>