ARM: shmobile: timer: Drop support for Cortex A8

Commit edf41009 ("ARM: shmobile: sh7372 dtsi: Remove Legacy
file") removed the DTS for the last shmobile SoC with a Cortex A8 CPU
core (sh7372 aka SH-Mobile AP4), hence drop support for it in the
loops-per-jiffy preset code.

As "div" is always 1 for supported contemporary ARM processors, we can
simplify the code:
  - Absorb shmobile_setup_delay_hz(), which was always called with
    mult = div = 1,
  - Return earlier if the Cortex A7/A15 arch timer exists and support is
    enabled.

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