mfd: syscon: Simplify syscon_match_pdevname()

Since we do not want to add new platform IDs for the syscon driver,
there is no reason to iterate over IDs. This patch simplifies
syscon_match_pdevname() function to remove such iteration.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>