ARM: shmobile: Add forward declaration of struct clk to silence warning

arch/arm/mach-shmobile/board-genmai-reference.c:23:0:
arch/arm/mach-shmobile/include/mach/clock.h:19:54: warning: 'struct clk' declared inside parameter list [enabled by default]
arch/arm/mach-shmobile/include/mach/clock.h:19:54: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>