ARM: shmobile: r8a7791: Fix HSUSB clock to hp_clk from mp_clk

HSUSB uses hp_clk rather than mp_clk for H/W register access.

Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
[horms: updated changelog]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>