ARM: dts: rockchip: Fix RTC description on rk3288-firefly-reload

ARM: dts: rockchip: Fix RTC description on rk3288-firefly-reload

Node names should be generic, so use 'rtc'.

Remove 'clock-frequency' as is not a valid property.

This fixes the following dt-schema warnings:

'hym8563@51' does not match '^rtc(@.*|-([0-9]|[1-9][0-9]+))?$'
Unevaluated properties are not allowed ('clock-frequency' was unexpected)

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://patch.msgid.link/20260303193855.828892-2-festevam@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>