rng: meson: make core clock optional

This fixes HWRNG support on Amlogic GXL, GXM, G12A, G12B & SM1
based boards dues to the lack of the core clock in the device tree.

It was reported breaking EFI boot in the Linux EFI stub, because the
EFI_RNG_PROTOCOL didn't check for the RNG device presence before
installing itself.

The Linux amlogic,meson-rng.yaml doesn't mandate the core clock,
this the clock should be ignores if not present.

Nevertheless, the clock should be present and this should be fixed
on the Linux meson-gxl.dtsi & meson-g12-common.dtsi then synced
with U-Boot.

The change has been tested on a Khadas VIM3, which uses the common
meson-g12-common.dtsi like the Odroid-C4 & Odroid-N2 in Scott's
report, along with the RNG cmd.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reported-by: Scott K Logan <logans@cottsay.net>
Fixes: bc40eb27 ("drivers/rng: add Amlogic hardware RNG driver")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Scott K Logan <logans@cottsay.net>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>