package/brcm-patchram-plus: fix build on sparc

On SPARC, the definitions of B2500000, B3000000, B3500000 and B4000000
are not necessarily available, so use those values only if defined in
the kernel headers.

It fixes SPARC build failures such as:
main.c:382:13: error: 'B2500000' undeclared here (not in a function)
  { 2500000, B2500000 },
             ^~~~~~~~
main.c:383:13: error: 'B3000000' undeclared here (not in a function)
  { 3000000, B3000000 },
             ^~~~~~~~
main.c:385:13: error: 'B3500000' undeclared here (not in a function)
  { 3500000, B3500000 },
             ^~~~~~~~
main.c:386:13: error: 'B4000000' undeclared here (not in a function)
  { 4000000, B4000000 }

Fixes:
 - http://autobuild.buildroot.org/results/f7012c08c935c3a6ccae50b84170190af5cd5cba



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>