ARM: shmobile: r8a7740: Fix TPU clock name

The TPU device is called renesas-tpu-pwm, not renesas_tpu_pwm. Fix the
clock name accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>