clk: renesas: r8a77990: Correct RCLK handling

According to R-Car Gen3 Hardware Manual Rev.1.00, R-Car E3 has the
RCLK Frequency Control Register (RCKCR), which determines the OSC and
RINT predivider values, and selection of the RCLK clock source between
RINT and the On-Chip Oscillator.

Hence change the OSC and RINT clock definitions to use the RCKCR
divider, and add the missing On-Chip Oscillator and RCLK clock source
switching logic.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>