regmap: include <linux/ktime.h> from include/linux/regmap.h

Similar to the readx_poll_timeout() macro calling ktime_* and using
ktime_t type, which is declared in <linux/ktime.h>. So, make
include/linux/regmap.h explicitly include <linux/ktime.h>, like
include/linux/iopoll.h does.  Otherwise, users of the macro will see
below errors.

error: implicit declaration of function ‘ktime_add_us’
[-Werror=implicit-function-declaration]

error: implicit declaration of function ‘ktime_get’
[-Werror=implicit-function-declaration]

error: implicit declaration of function ‘ktime_compare’
[-Werror=implicit-function-declaration]

include/linux/regmap.h:128:2: error: unknown type name ‘ktime_t’
  ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>