MIPS: ath79: select HAVE_CLK

MIPS: ath79: select HAVE_CLK

It is needed in order to get rid of the following errors:

arch/mips/ath79/clock.c:353:13: error: redefinition of 'clk_get'
include/linux/clk.h:281:27: note: previous definition of 'clk_get' was here
arch/mips/ath79/clock.c:377:5: error: redefinition of 'clk_enable'
include/linux/clk.h:295:19: note: previous definition of 'clk_enable' was here
arch/mips/ath79/clock.c:383:6: error: redefinition of 'clk_disable'
include/linux/clk.h:300:20: note: previous definition of 'clk_disable' was here
arch/mips/ath79/clock.c:388:15: error: redefinition of 'clk_get_rate'
include/linux/clk.h:302:29: note: previous definition of 'clk_get_rate' was here
arch/mips/ath79/clock.c:394:6: error: redefinition of 'clk_put'
include/linux/clk.h:291:20: note: previous definition of 'clk_put' was here

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4170/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>