clk: sunxi-ng: fix build failure in ccu-sun9i-a80 driver

The ccu-sun9i-a80 driver uses the ccu_mult_ops struct, but unlike the other
users it doesen't select the corresponding Kconfig symbol under which the
struct is compiled in.

This results in the following link error with CONFIG_SUN9I_A80_CCU=y and
CONFIG_SUNXI_CCU_MULT=n:

drivers/built-in.o:(.data+0x2d638): undefined reference to 'ccu_mult_ops'

Fix this by explicitly selecting CONFIG_SUNXI_CCU_MULT like the other
users of the struct.

Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>