rockchip: adc: rockchip-saradc: use dev_read_addr_ptr

rockchip: adc: rockchip-saradc: use dev_read_addr_ptr

The fdt_addr_t and phys_addr_t size have been decoupled.
A 32bit CPU can expext 64-bit data from the device tree parser,
so use dev_read_addr_ptr in the rockchip-saradc.c file.
As we are there also streamline the error response to -EINVAL on return.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>