AON blocks in core devicetrees have base-address marked as mem instead of control

[sctincman]

Most parent blocks appear to reference the block base-address via regs: <{addr} {size}> and mark them as such via reg-names: "control", however the aon block in the FE310/FS510 trees instead refers to its base as reg-names: "mem".

This doesn't affect rtc and wdog as they are derived from the aon block itself and simply use the base-address emitted, but the lfclk/lfrosc entry/device in freedom-devicestree-tools specifically matches on the reference's control reg-name. As aon here doesn't define control this will return 0 and cause an incorrect entry for the two register addresses used in lfclk/lfrosc, which is partially responsible for sifive/freedom-metal#238 (get_rate_hz on lfclk returns 0).

reg-name: "control" here seems appropriate and used in other blocks (and mem appears to be reserved for things like DTIM/ITIM).

Affected DTS:
bsp/qemu-sifive-e31/core.dts
bsp/qemu-sifive-s51/core.dts
bsp/sifive-hifive1/core.dts
bsp/sifive-hifive1-revb/core.dts