clk: bcm2835: Constify struct debugfs_reg32

clk: bcm2835: Constify struct debugfs_reg32

bcm2835_debugfs_clock_reg32 is never changed and can therefore be made
const.

This allows the compiler to put it in the text section instead of the
data section.

Before:
   text    data     bss     dec     hex filename
  26598   16088      64   42750    a6fe drivers/clk/bcm/clk-bcm2835.o

After:
   text    data     bss     dec     hex filename
  26662   16024      64   42750    a6fe drivers/clk/bcm/clk-bcm2835.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lkml.kernel.org/r/20200508220238.4883-1-rikard.falkeborn@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>