serial: sifive: Change include order

Like clk_get_by_index, there is requirement for
clk_get_by_index_nodev. In this case to make common
code functionalities for dev and nodev, clk_get_by_index
is trying to get the index of clock by passing ofnode
instead of actual dev like current gpio uclass does.

In these scenarios with current order of include files
the serial_sifive driver is unable to find CONFIG_ENV_SIZE.

In file included from arch/riscv/include/asm/u-boot.h:23:0,
                 from include/dm/of.h:10,
                 from include/dm/ofnode.h:12,
                 from include/clk.h:11,
                 from drivers/serial/serial_sifive.c:6:
include/environment.h:145:19: error: 'CONFIG_ENV_SIZE'
undeclared here (not in a function); did you mean 'CONFIG_CMD_XIMG'?
 #define ENV_SIZE (CONFIG_ENV_SIZE - ENV_HEADER_SIZE)

So, fix consists of changing the order of include files
in serial_sifive.c to include first common.h file.

Cc: Anup Patel <Anup.Patel@wdc.com>
Cc: Rick Chen <rick@andestech.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>