powerpc: fix dead default for SYS_L3_SIZE

powerpc: fix dead default for SYS_L3_SIZE

The 'default ... if SYS_L3_SIZE_512KB' statement is written twice, making
the second instance dead code.

Based on the context, I believe the author of the code intended to set a
default value of '1048576' when SYS_L3_SIZE_1024KB is set.

This dead code was detected by kconfirm, a static analysis tool for
Kconfig.

Fixes: b85d75951fe ("powerpc: Migrate SYS_L3_SIZE to Kconfig")
Signed-off-by: Julian Braha <julianbraha@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>