drivers: serial: nrfx: Update Kconfig dependencies for RX cache

Two Kconfig symbols were tied to a board-specific DT nodelabel - namely
the `NRFX_UARTE_CONFIG_RX_CACHE_ENABLED` and `UART_<n>_RX_CACHE_SIZE`
were tied to `shared_ram3x_region` - but this is not necessary.

That DT node is where the UARTE driver RX/TX cache buffers are placed
in the default nRF54H20 DK memory map, but on another board, they could
be located somewhere else.

The exact memory sub-regions used for this purpose are indicated using
the `memory-regions` property on each UARTE instance, which should be
generic enough already, so let's use that instead.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>