xtensa: Remove local no-op dram_init_banksize()

When this no-op dram_init_banksize() is removed, the weak default will
be used instead, which correctly sets the bi_dram[] banksize values.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Ovidiu Panait <ovidiu.panait@windriver.com>