i2c: xgene-slimpro: include linux/io.h for memremap

i2c: xgene-slimpro: include linux/io.h for memremap

The newly added support for the pcc mailbox fails to build
in some configurations:

drivers/i2c/busses/i2c-xgene-slimpro.c: In function 'xgene_slimpro_i2c_probe':
drivers/i2c/busses/i2c-xgene-slimpro.c:516:25: error: implicit declaration of function 'memremap'; did you mean 'memcmp'? [-Werror=implicit-function-declaration]
drivers/i2c/busses/i2c-xgene-slimpro.c:518:13: error: 'MEMREMAP_WB' undeclared (first use in this function)
drivers/i2c/busses/i2c-xgene-slimpro.c:518:13: note: each undeclared identifier is reported only once for each function it appears in

This includes the missing header file.

Fixes: df5da47fe722 ("i2c: xgene-slimpro: Add ACPI support by using PCC mailbox")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>