jz4740-battery: Should include linux/io.h

During test-build (with disabled 'depends on') I found that
jz4740-battery driver lacks linux/io.h, which makes build break
like this (on x86):

  CC [M]  drivers/power/isp1704_charger.o
jz4740-battery.c: In function 'jz_battery_read_voltage':
jz4740-battery.c:84: error: implicit declaration of function 'readw’
jz4740-battery.c: In function 'jz_battery_probe':
jz4740-battery.c:284: error: implicit declaration of function 'ioremap_nocache’
jz4740-battery.c:285: warning: assignment makes pointer from integer without a cast
jz4740-battery.c:372: error: implicit declaration of function 'iounmap'
make[2]: *** [drivers/power/jz4740-battery.o] Error 1

This patch fixes the issues, and thus makes it easier to build-test
the driver for me.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>