ARM: shmobile: Setup r8a7790 arch timer based on MD pins

Update the r8a7790 arch timer setup code to configure the
frequency dynamically at boot time. This means that the arch
timer driver will be able to detect a timer frequency that
has been calculated based on the MD pins instead of a fixed
and potentially incorrect 13 MHz.

With this patch applied the Linux kernel will correctly
support the r8a7790 Lager board that uses a 20 Mhz EXTAL.
The arch timer will operate on 10 MHz and the Linux arch
timer driver will be correctly configured to use 10 MHz.

Without this patch the 20 MHz EXTAL will be used to drive
the arch timer at 10 MHz, but the Linux arch timer driver
will believe it is counting at 13 Mhz.

Reported-by: Ulrich Hecht <ulrich.hecht@gmail.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Ulrich Hecht <ulrich.hecht@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>